Roast Beef and Provolone Panini with Caramelized Onions and Horseradish Mayo

Name: Roast Beef and Provolone Panini with Caramelized Onions and Horseradish Mayo
Prep time: 10 min
Cook time: 52 min
Total time: 1 h 2 min
Yield: 4 servings
Cuisine: American
Ingredients: 2 large (about 14 oz total) (400 g) large yellow onions, halved, peeled and sliced into ¼-inch (6 mm) half-moons; 1 tbsp (15 ml) olive oil; 1 tbsp (14 g) unsalted butter; ¼ tsp (1½ g) kosher salt; 2 tbsp (30 ml) water, added 1 tbsp at a time, as needed, to deglaze; 1 tsp (5 ml) balsamic vinegar; ½ cup (110 g) mayonnaise; 2 tbsp (30 g) prepared horseradish, drained of excess liquid; 1 tsp (5 g) Dijon mustard; 1 tsp (5 ml) fresh lemon juice; freshly ground black pepper, to taste, about ¼ tsp; 4 rolls (about 4 oz each) (450 g) ciabatta rolls, split horizontally with a serrated knife; 12 oz (340 g) thinly sliced deli roast beef, at room temperature, loosely folded or ruffled; 8 slices (about 6 oz) (170 g) sliced provolone cheese, at room temperature; 2 tbsp (30 ml) olive oil, for brushing the outside of the bread

From the MyKitchenBank Kitchen

Thinly sliced roast beef, melty provolone, jammy caramelized onions and a sharp horseradish mayo are pressed between crisp, olive-oil-brushed ciabatta. It's a deli-style sandwich that's easy to make at home, with a crunchy crust and a gooey, savory center.

Prep: 10 min
Cook: 52 min
Wait: 3 min
Total: 1 hr 5 min
Makes: 4 paninis

Jump to recipe ♡ Save

Ingredients

4 servings

For the caramelized onions
2 large (about 14 oz total) (400 g) large yellow onions, halved, peeled and sliced into ¼-inch (6 mm) half-moons
1 tbsp (15 ml) olive oil
1 tbsp (14 g) unsalted butter
¼ tsp (1.5 g) kosher salt
2 tbsp (30 ml) water, added 1 tbsp at a time, as needed, to deglaze
1 tsp (5 ml) balsamic vinegar
For the horseradish mayo
½ cup (110 g) mayonnaise
2 tbsp (30 g) prepared horseradish, drained of excess liquid
1 tsp (5 g) Dijon mustard
1 tsp (5 ml) fresh lemon juice
freshly ground black pepper, to taste, about ¼ tsp
For the paninis
4 rolls (about 4 oz each) (450 g) ciabatta rolls, split horizontally with a serrated knife
12 oz (340 g) thinly sliced deli roast beef, at room temperature, loosely folded or ruffled
8 slices (about 6 oz) (170 g) sliced provolone cheese, at room temperature
2 tbsp (30 ml) olive oil, for brushing the outside of the bread

Equipment (8)

Panini press, or a 12-inch (30 cm) heavy skillet plus a second heavy skillet or foil-wrapped brick for weight
12-inch (30 cm) skillet for the onions (stainless steel or cast iron)
Baking sheet with a wire rack (for keeping the first batch warm)
Cutting board and chef's knife
Serrated bread knife
Small mixing bowl
Pastry brush or spoon
Wooden spoon or heatproof spatula

Steps

Before you start

Start the onions first. They need 35 to 40 minutes, and you make the mayo, prep the bread and preheat the press while they cook. Prep (slicing and assembling) is 10 minutes of the total; the rest overlaps with the onions.
Take the roast beef and provolone out of the fridge 15 minutes before assembling. Room-temperature fillings heat through faster, so the cheese melts before the bread over-browns.
Most presses and 12-inch skillets fit only 2 sandwiches at a time, so plan on 2 batches. The first batch waits in a 200°F (95°C) oven until the second batch is done, then everything is served together.
If you have no press, you'll need a second heavy skillet or a foil-wrapped brick to weigh the sandwiches down.

1
Slice the onions
- Halve the 2 large yellow onions through the root, peel them, and slice them into ¼-inch (6 mm) half-moons.
- Slices of even thickness cook at the same rate.
+ Show details− Hide details
Ready when: Slices are about ¼ inch thick and separated into half-moons.
2
Caramelize the onions
- Heat 1 tbsp olive oil and 1 tbsp butter in a 12-inch (30 cm) skillet over medium-low heat until the butter foams.
- Add the onions and ¼ tsp salt and stir to coat.
- Cook for 35 to 40 minutes, stirring every 3 to 4 minutes at first and then every minute or two as they darken.
- If the pan looks dry or browned bits start to scorch, add 1 tbsp water (up to 2 tbsp total) and scrape the bottom.
- Stir in the balsamic vinegar for the last minute.
- Start the mayo, bread prep and preheating (next steps) while the onions cook.
+ Show details− Hide details
Ready when: Onions are deep golden brown, very soft and jammy, and reduced to about ⅔ cup (about 120 g) total.
Tip: Don't rush with higher heat. Onions that brown too fast turn bitter instead of sweet.
3
Mix horseradish mayo
- While the onions cook, stir together the mayonnaise, drained horseradish, Dijon mustard, lemon juice and a few grinds of black pepper in a small bowl.
- Taste it: it should have a clear sinus-tingling kick.
- Add another teaspoon of horseradish if you want it hotter.
- Refrigerate until needed.
+ Show details− Hide details
Ready when: Mayo is smooth, evenly pale and tastes sharp and tangy.
Tip: Squeeze the horseradish in a fine strainer or paper towel first. Extra liquid makes the mayo runny and the bread soggy.
4
Prep the bread
- Split each ciabatta roll horizontally with a serrated knife.
- If the rolls are very thick, gently pull out a little of the soft crumb from the cut sides so the sandwich presses flat.
- Brush the outer crusts of all 4 rolls (top and bottom) with 2 tbsp olive oil in total.
+ Show details− Hide details
Ready when: All outer surfaces have a thin, even sheen of oil, with no pools.
Tip: Oil the outside, not the inside. Oiled outsides crisp and brown, and dry insides keep the mayo from soaking in.
5
Preheat press and ovenOven 200°F / 95°C
- During the last 5 minutes of onion cooking, heat the panini press to about 375°F (190°C).
- If using a skillet, set a second 12-inch (30 cm) skillet over medium-low heat for 4 to 5 minutes.
- At the same time, heat the oven to 200°F (95°C) and put a wire rack set on a baking sheet inside to keep the first batch warm.
+ Show details− Hide details
Ready when: The press indicator light shows ready, or the skillet is hot enough that a drop of water sizzles away within a couple of seconds.
6
Assemble the paninis
- Spread about 2 tsp horseradish mayo on the cut side of each roll half (8 halves), saving the remaining mayo for dipping.
- On each bottom half, layer 1 slice provolone, about 3 oz (85 g) of roast beef folded in loose ruffles, a quarter of the caramelized onions (about 2½ tbsp), then a second slice of provolone.
- Close with the top halves, mayo side down.
- Cheese on both sides of the beef and onions helps glue the sandwich together.
+ Show details− Hide details
Ready when: Each sandwich is stacked evenly, with fillings no wider than the bread.
Tip: Ruffle the beef instead of laying it flat. It heats evenly and gives the sandwich a better bite.
7
Press the first batchOven 200°F / 95°C
- Place 2 sandwiches in the press and close the lid gently, without forcing it.
- Cook for 4 to 5 minutes.
- If using a skillet, add 2 sandwiches to the medium-low skillet, set a heavy skillet on top for weight, and cook 3 to 4 minutes per side, flipping once.
- Move the finished sandwiches to the rack in the 200°F (95°C) oven and leave them there until the second batch is done.
+ Show details− Hide details
Ready when: The bread is deep golden with dark grill marks, feels crisp when tapped, and melted cheese is just starting to ooze at the edges.
Tip: If the bread browns before the cheese melts, lower the heat and press for another minute.
8
Press the second batchOven 200°F / 95°C
- Cook the remaining 2 sandwiches the same way: 4 to 5 minutes in the press, or 3 to 4 minutes per side in the medium-low skillet.
- Leave the first batch in the oven during this time.
- When the second batch is done, turn off the oven and take out the first batch.
+ Show details− Hide details
Ready when: Bread is deeply golden and crisp, with cheese melted and oozing at the edges.
9
Rest, slice and serve
- Let all 4 paninis rest on the board for 2 to 3 minutes so the molten cheese sets slightly.
- Cut each in half diagonally with a serrated knife, using a gentle sawing motion.
- Serve right away, with the reserved horseradish mayo for dipping.
+ Show details− Hide details
Ready when: Cut halves hold together, with stretchy cheese and visible layers of beef and onion.
Tip: Cutting right out of the press makes the filling slide out and can burn your mouth. Waiting a couple of minutes helps.

Good to know

Tips

Use freshly sliced deli roast beef if you can. Pre-packaged slices are often thicker and chewier, and they don't ruffle as well.
Don't overfill. About 3 oz (85 g) of beef per sandwich is enough, and more stops the cheese from melting through and makes pressing messy.
Press gently at first. Squashing the bread too hard flattens the crumb and pushes the filling out the sides.
If the onions are done before you're ready, take them off the heat and keep them covered. They reheat in seconds.

Make ahead

Caramelized onions can be made up to 4 days ahead and the horseradish mayo up to 3 days ahead; refrigerate both in airtight containers. Bring the onions to room temperature or warm them briefly before assembling. Paninis are best pressed just before serving.

Storage & reheating

Room temperature: up to 2 hours. Fridge: wrap leftover pressed paninis tightly and keep up to 2 days. Freezer: wrap individually in foil and freeze up to 1 month. Reheat refrigerated paninis in a 350°F (175°C) oven for 8 to 10 minutes, or in a skillet over medium-low heat for 3 to 4 minutes per side, until hot in the center and crisp outside. Reheat frozen ones, still wrapped, at 350°F (175°C) for 25 to 30 minutes, then unwrap and crisp for 5 minutes.

Substitutions

ciabatta rolls → Sourdough slices or a 16-inch (40 cm) ciabatta loaf cut into 4 pieces. Use 8 slices of sourdough, about ½ inch (1.25 cm) thick. A loaf cut into 4 portions works in place of rolls.
sliced provolone cheese → Swiss, Gruyère or white cheddar. All melt well. Gruyère adds a nuttier flavor, and sharp white cheddar gives more tang.
prepared horseradish → Whole-grain mustard, or a pinch of wasabi paste. Use 1 tbsp whole-grain mustard for a milder mayo. Add wasabi gradually, since it varies in strength.
olive oil → Softened unsalted butter. Use 2 tbsp spread thinly on the outsides for a richer, more golden crust.