Croissant Breakfast Sandwich Bake

Name: Croissant Breakfast Sandwich Bake
Prep time: 20 min
Cook time: 45 min
Total time: 1 h 5 min
Yield: 8 servings
Cuisine: American
Ingredients: 6 whole croissants, day-old, split horizontally and cut into 1-inch (2.5 cm) thick slices; 8 oz (225 g) sliced deli ham, cut into strips or rough pieces; 1½ cups (180 g) Gruyère cheese, block, coarsely shredded; 8 whole large eggs, cold, directly from the refrigerator; 2 cups (480 ml) whole milk, cold, straight from the refrigerator; 1 tbsp (15 g) Dijon mustard; 1 tsp (6 g) kosher salt; ½ tsp (1 g) freshly ground black pepper; ¼ tsp (½ g) freshly grated nutmeg; 1 tbsp (14 g) unsalted butter, soft, for greasing the dish; 4 sprigs fresh thyme sprigs, leaves only (optional)

From the MyKitchenBank Kitchen

A savory breakfast casserole layered with buttery croissants, sliced ham, and melted Gruyère cheese, bound together with a custardy egg mixture. Assemble the night before and bake in the morning for an impressive, golden-brown dish that's rich enough to serve as the centerpiece of brunch.

Prep: 20 min
Cook: 45 min
Wait: 8 hr
Total: 9 hr 5 min
Makes: 1 baking dish (8 servings)

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Steps

Before you start

Read through the recipe; this dish needs 8 hours (or overnight) chilling before baking
Use day-old croissants if possible—they absorb the custard better than fresh ones
Have all ingredients at the temperature listed—cold eggs and milk emulsify better into a smooth custard

1
Prepare the baking dish
- Lightly butter the inside of a 9×13-inch (23×33 cm) baking dish with the soft butter, coating the bottom and sides evenly.
- Set aside.
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Ready when: Dish is evenly coated with a thin layer of butter
2
Layer croissant and ham
- Arrange half of the croissant slices in the bottom of the prepared dish in a single layer, slightly overlapping if needed.
- Scatter half of the ham pieces over the croissants, followed by half of the shredded Gruyère.
- Repeat with the remaining croissant slices, ham, and cheese, ending with cheese on top.
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Ready when: All ingredients are evenly layered and filling the dish
Tip: Stagger the croissant slices slightly so the custard can flow through the layers—don't pack them too tightly
3
Make the custard mixture
- In a large mixing bowl, whisk together the 8 cold eggs and 2 cups cold milk until completely smooth, 1–2 minutes.
- Add the Dijon mustard, kosher salt, black pepper, and nutmeg.
- Whisk again until fully combined and no streaks of mustard remain.
- The mixture should be pale and homogeneous.
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Ready when: Custard is smooth, no lumps or streaks visible
Tip: Whisk thoroughly until the mustard is fully dispersed—bits of mustard can cause grainy patches in the baked custard
4
Pour custard over layers
- Slowly pour the custard mixture evenly over the layered croissants, ham, and cheese, covering all the dry spots.
- Let it settle for about 1 minute, then gently press down on the top layer with the back of a spoon to ensure the custard soaks through.
- If any dry spots remain, pour a few more tablespoons into those areas.
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Ready when: All croissant pieces are moist and custard covers the surface
Tip: Pour slowly to avoid splashing, and press gently—too much force can compress the croissants and make them dense
5
Cover and chill overnight
- Cover the baking dish tightly with plastic wrap and refrigerate for at least 8 hours, or up to 24 hours.
- The custard will continue to soak into the croissants during this time, and the flavors will meld.
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Ready when: Casserole is fully chilled and ready to bake
Tip: This make-ahead step is essential—it allows the croissants to absorb the custard fully, ensuring a creamy interior
6
Remove from refrigerator and preheatOven 350°F / 175°C
- Remove the baking dish from the refrigerator and let it sit on the counter for 15 minutes while you preheat the oven to 350°F (175°C).
- The casserole should still be cold when it goes into the oven, but bringing it to room temperature slightly helps it cook more evenly.
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Ready when: Oven is fully preheated to 350°F (175°C)
Tip: Don't skip the brief rest—it allows the casserole to cook more evenly from edge to center
7
Bake until golden and setOven 350°F / 175°C
- Place the casserole in the center of the preheated 350°F (175°C) oven.
- Bake for 40–45 minutes, until the top is deep golden brown and a knife inserted into the center comes out clean or with just a few moist crumbs clinging to it.
- The edges will puff up slightly, and the center should jiggle only very slightly when gently shaken.
- If the top is browning too quickly before the center is set, tent loosely with foil for the final 10 minutes.
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Ready when: Top is deep golden brown, knife inserted in center comes out clean or nearly clean, center barely jiggles
Tip: Overbaking will make the custard rubbery; stop as soon as the center is just set—it will continue to cook slightly as it cools
8
Rest before serving
- Remove the casserole from the oven and let it rest on the counter for 10 minutes.
- This allows the eggs to finish setting gently and makes the dish easier to portion cleanly.
- Scatter the fresh thyme leaves (if using) over the top just before serving for a hint of herbal freshness.
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Ready when: Casserole is still warm, thyme garnish is in place
Tip: Don't skip this rest—cutting too soon will cause the casserole to fall apart

Good to know

Tips

Day-old croissants work best because fresh ones are too delicate and will fall apart when soaked. If your croissants are fresh, let them sit uncovered at room temperature for a few hours to firm up slightly.
For extra richness, add ½ cup grated Gruyère and ½ cup freshly grated Parmesan to the egg mixture itself before pouring—this gives the custard even more flavor.
If you'd like more vegetables, sauté 1 cup sliced mushrooms or 1 bunch thinly sliced scallions until tender, then add them to the layers between the ham and cheese.
This casserole serves 8 as a main dish with sides, or 10–12 as part of a larger brunch spread. The recipe easily doubles in two 9×13-inch dishes for a crowd.

Make ahead

Assemble and chill the casserole up to 24 hours in advance. Bake fresh on the morning you plan to serve it. The unbaked, chilled casserole can also be frozen (in the baking dish, covered tightly with plastic wrap and foil) for up to 2 weeks. Thaw in the refrigerator overnight, then bake as directed, adding 5–10 minutes to the baking time if still very cold.

Storage & reheating

Room temperature: up to 2 hours. Refrigerator: cover and store for up to 3 days; reheat, covered, in a 300°F (150°C) oven for 15–20 minutes until warmed through. Freezer: wrap leftover portions tightly in foil or freezer-safe containers for up to 1 month; reheat from frozen in a 300°F (150°C) oven for 25–30 minutes, covered.

Substitutions

Gruyère cheese, block → sharp white cheddar or Emmental. These cheeses melt smoothly and provide similar nutty flavor; use the same amount
sliced deli ham → crispy cooked bacon, chopped, or smoked salmon. Bacon adds smokiness; use 6 slices cooked and crumbled. Salmon works for a more elegant brunch
whole milk, cold → heavy cream mixed with water (1:1 ratio). This makes a richer custard; use 1 cup cream plus 1 cup water
croissants, day-old → brioche buns or thick-cut challah. Brioche and challah are slightly denser but absorb custard well; use the same amount