Grilled Balsamic Portobello Mushroom Burgers with Provolone and Spinach

Name: Grilled Balsamic Portobello Mushroom Burgers with Provolone and Spinach
Prep time: 15 min
Cook time: 18 min
Total time: 33 min
Yield: 4 servings
Cuisine: Mediterranean
Ingredients: 4 caps (450 g) large portobello mushroom caps (about 4 inches / 10 cm wide), stems removed, wiped clean with a damp paper towel; 3 tbsp (45 ml) balsamic vinegar; 3 tbsp (45 ml) extra-virgin olive oil; 2 cloves (8 g) garlic, finely minced; 1 tbsp (15 ml) soy sauce; 1 tsp (2 g) fresh thyme leaves, stripped from stems and chopped; ½ tsp (3 g) kosher salt; ¼ tsp (½ g) black pepper, freshly ground; 1 small (100 g) small red onion, peeled and cut into four ½-inch (1.25 cm) thick rounds; 1 tbsp (15 ml) neutral oil such as canola or avocado, plus a little extra on a folded paper towel for the grates; 4 slices (100 g) sliced provolone cheese, at room temperature; 2 cups, packed (60 g) baby spinach, washed and dried well; 4 tbsp (60 g) mayonnaise; 1 tsp (5 g) Dijon mustard; 4 buns (280 g) hamburger buns, split; 1 medium (150 g) ripe tomato, cut into 4 slices about ¼ inch (6 mm) thick (optional)

From the MyKitchenBank Kitchen

Meaty portobello caps soak up a garlicky balsamic marinade, then char on the grill until juicy and tender. Melted provolone, fresh spinach, and sweet grilled red onion on a toasted bun make a vegetarian burger that satisfies like the classic.

Prep: 15 min
Cook: 18 min
Wait: 30 min
Total: 1 hr 3 min
Makes: 4 burgers

Save to My Kitchen Jump to recipe

Ingredients

4 servings

For the mushrooms
4 caps (450 g) large portobello mushroom caps (about 4 inches / 10 cm wide), stems removed, wiped clean with a damp paper towel
3 tbsp (45 ml) balsamic vinegar
3 tbsp (45 ml) extra-virgin olive oil
2 cloves (8 g) garlic, finely minced
1 tbsp (15 ml) soy sauce
1 tsp (2 g) fresh thyme leaves, stripped from stems and chopped
½ tsp (3 g) kosher salt
¼ tsp (0.5 g) black pepper, freshly ground
For assembly
1 small (100 g) small red onion, peeled and cut into four ½-inch (1.25 cm) thick rounds
1 tbsp (15 ml) neutral oil such as canola or avocado, plus a little extra on a folded paper towel for the grates
4 slices (100 g) sliced provolone cheese, at room temperature
2 cups, packed (60 g) baby spinach, washed and dried well
4 tbsp (60 g) mayonnaise
1 tsp (5 g) Dijon mustard
4 buns (280 g) hamburger buns, split
1 medium (150 g) ripe tomato, cut into 4 slices about ¼ inch (6 mm) thick (optional)

Equipment (7)

Gas or charcoal grill with a lid
Grill tongs and a wide metal spatula
Shallow baking dish or 9×13-inch (23×33 cm) pan for marinating
Small bowl and whisk
Pastry brush
Cutting board and chef's knife
Toothpicks (optional, to hold onion rings together)

Steps

Before you start

Choose portobello caps about 4 inches (10 cm) wide so they fit the buns; they should feel firm and dry, not slimy.
The caps need at least 30 minutes in the marinade, so start there. Use that time to preheat the grill and slice the onion.
Have all toppings and buns within reach of the grill. Everything moves quickly once the caps come off the heat.
Do not rinse the mushrooms under running water. They absorb it and turn soggy on the grill.

1
Prepare the caps
- Snap or cut the stems off the portobello caps.
- Wipe the tops and gills with a damp paper towel to remove any dirt.
- If you like a cleaner look and want to avoid dark juices on the bun, scrape out the gills with a spoon.
- Lay the caps gill-side up in a shallow baking dish in a single layer.
+ Show details− Hide details
Ready when: Caps are clean, stemless, and sit flat in the dish.
Tip: Never soak mushrooms in water. They act like sponges and will steam instead of char.
2
Whisk the marinade
- In a small bowl whisk together the balsamic vinegar, olive oil, minced garlic, soy sauce, thyme, salt, and pepper until the mixture looks cloudy and slightly thickened.
- Spoon or brush it over the gill sides first, letting it pool in the cavities, then brush the tops.
+ Show details− Hide details
Ready when: Every cap is glossy on both sides with no dry patches.
3
Marinate the mushrooms
- Leave the caps gill-side up at room temperature for 30 minutes, flipping them once at the 15-minute mark and spooning any marinade from the dish back over them.
- While they rest, preheat the grill (next step) and slice the onion.
+ Show details− Hide details
Ready when: Caps look darker and slightly shrunken, and most of the marinade has soaked in.
Tip: Do not marinate longer than 1 hour. The vinegar will make the caps limp and overly salty.
4
Preheat and oil grillOven 450°F / 230°C
- About 10 minutes before the marinating time ends, heat the grill to medium-high, 400–450°F (200–230°C), with the lid closed.
- Scrub the grates with a grill brush.
- Fold a paper towel, dip it in a little of the neutral oil, grip it with tongs, and wipe the hot grates.
+ Show details− Hide details
Ready when: A grill thermometer reads 400–450°F (200–230°C) and the grates look clean and slightly glossy.
Tip: Clean, oiled, hot grates stop the caps from sticking and tearing.
5
Grill the onions
- Brush the onion rounds on both sides with the remaining neutral oil.
- Push a toothpick through each round to hold the rings together if you wish.
- Place them on the grill over direct heat and cook with the lid closed for 4 minutes per side, turning once.
+ Show details− Hide details
Ready when: Onions have dark grill marks and are tender when pierced with a fork but still hold their shape.
Tip: Remove toothpicks before serving.
6
Grill the caps
- Lift the caps from the marinade, letting excess drip off, and reserve the leftover marinade.
- Place the caps gill-side down on the grill beside the onions.
- Grill with the lid closed for 5 minutes, then flip and brush the gill sides with the reserved marinade.
- Cook 5 to 6 minutes more.
+ Show details− Hide details
Ready when: Caps are deeply browned with grill marks, have shrunk about a quarter, and a knife slides easily into the thickest part. Juices pool in the cavity.
Tip: Do not press the caps with a spatula, which squeezes out their juices and makes the burgers dry.
7
Melt the provolone
- Flip the caps so they are gill-side up.
- Drape one provolone slice over each cap, close the lid, and cook for 1 to 2 minutes, just until the cheese begins to melt at the edges.
- Move the caps and onions to a plate.
+ Show details− Hide details
Ready when: Cheese is soft, glossy, and drooping over the edges of the cap.
Tip: Watch closely. Cheese goes from melted to scorched quickly on a hot grill.
8
Toast the buns
- Place the split buns cut-side down on the cooler edge of the grill, or directly on the grates if the heat is lowered, and toast for 1 to 2 minutes.
- Meanwhile stir the mayonnaise and Dijon together in a small bowl.
+ Show details− Hide details
Ready when: Cut sides are golden brown with light grill marks.
Tip: Do not walk away. Buns burn in seconds.
9
Assemble the burgers
- Spread the mustard mayonnaise on both halves of each toasted bun.
- Pile a small handful of spinach (about ½ cup / 15 g) on each bottom bun, add a tomato slice if using, then set a cheese-topped portobello cap on top.
- Finish with a grilled onion round and the top bun.
+ Show details− Hide details
Ready when: Burgers stand tall and the spinach is just starting to wilt from the warm cap.
Tip: Placing the spinach under the cap protects the bun from mushroom juices.
10
Rest and serve
- Let the assembled burgers sit for 2 minutes so the juices settle into the bun and the cheese sets slightly.
- Serve right away with napkins.
+ Show details− Hide details
Ready when: Burgers feel warm and hold together when gently pressed.

Good to know

Tips

Cook the caps mostly gill-side down first: it sears the surface and keeps the juices inside the cap instead of running into the fire.
Choose caps of similar size so they all finish at the same time.
No grill? Cook the caps and onions in a hot cast-iron grill pan over medium-high heat for the same times, or roast at 425°F (220°C) for 18 to 20 minutes.
Season lightly with salt at the start. The soy sauce and cheese add saltiness, and mushrooms shrink as they cook.

Make ahead

"Marinate the caps in the refrigerator, covered, for up to 4 hours (bring to room temperature 20 minutes before grilling). Mix the mustard mayonnaise up to 3 days ahead and keep it chilled."

Storage & reheating

"Best eaten immediately. Store cooked caps and onions separately from buns in an airtight container in the refrigerator for up to 3 days. Reheat caps in a 350°F (175°C) oven or a skillet over medium heat for 5 to 6 minutes until hot. Cooked caps can be frozen for up to 2 months, though their texture will be softer. Assembled burgers do not keep well at room temperature beyond 2 hours."

Substitutions

sliced provolone cheese → Swiss, fontina, mozzarella, or a vegan melting cheese. Any good melter works; Swiss gives a nuttier flavor.
baby spinach → Arugula. Adds a peppery bite; use the same amount.
mayonnaise → Vegan mayonnaise or pesto. Use vegan mayo with vegan cheese and buns for a fully plant-based burger.
hamburger buns → Gluten-free buns or large lettuce leaves. Toast gluten-free buns lightly as they dry out quickly.
soy sauce → Tamari. Makes the burger gluten-free.