Ham, Leek, and Gruyère Croissant Bake

Why You’ll Love This Recipe

• Golden, flaky croissant edges with a custardy, melty center.
• Smart way to use day-old croissants and leftover ham.
• Dijon and thyme add bistro-level flavor with minimal effort.
• Easy to assemble ahead for stress-free brunches and holidays.

Grocery List

• Produce: 2 medium leeks, fresh thyme, chives or parsley (for garnish)
• Dairy: Eggs, whole milk, heavy cream, unsalted butter, Gruyère cheese
• Pantry: Day-old croissants, ham (deli or leftover), Dijon mustard, fine sea salt, black pepper, ground nutmeg

Full Ingredients

For the croissant base

• 6 large day-old croissants (about 18–20 oz/510–570 g), torn into 2-inch pieces
• 1 tbsp unsalted butter (for sautéing)
• 2 medium leeks (about 12 oz/340 g), white and light green parts only, thinly sliced and well rinsed
• 8 oz (225 g) thick-cut cooked ham, diced into 1/2-inch pieces
• 8 oz (225 g) Gruyère cheese, coarsely shredded (reserve 2 oz/55 g for topping)

For the Dijon-thyme custard

• 6 large eggs
• 2 cups (480 ml) whole milk
• 1/2 cup (120 ml) heavy cream
• 2 tbsp Dijon mustard
• 1 tbsp fresh thyme leaves (or 1 tsp dried thyme)
• 1 tsp fine sea salt
• 1/2 tsp freshly ground black pepper
• Pinch ground nutmeg (optional but lovely)

To finish

• Chopped chives or flat-leaf parsley, for garnish

Step-by-Step Instructions

Step 1: Heat the oven and prep the dish

Preheat the oven to 350°F (175°C) with a rack in the center. Lightly grease a 9×13-inch (3 L) baking dish with butter or nonstick spray. Tear the day-old croissants into 2-inch pieces; set aside.

Step 2: Sauté the leeks until tender

In a large skillet over medium heat, melt 1 tbsp butter. Add the sliced, well-rinsed leeks and a pinch of salt. Cook, stirring occasionally, until softened and sweet but not browned, 6 to 8 minutes. Remove from heat and cool slightly.

Step 3: Whisk the Dijon-thyme custard

In a large bowl, whisk the eggs until smooth. Add milk, cream, Dijon mustard, thyme, salt, pepper, and nutmeg; whisk until fully combined and no egg streaks remain. This is your savory custard.

Step 4: Assemble the layers

Scatter half of the torn croissants in the prepared dish. Top with half the ham, half the sautéed leeks, and half the shredded Gruyère. Repeat with the remaining croissants, ham, leeks, and 4 oz (115 g) more Gruyère (reserve 2 oz/55 g for the top). Slowly pour the custard evenly over everything, letting it seep into the layers. Press down gently with your palms or the back of a spoon so the croissants absorb the custard. Let soak for 15 minutes at room temperature (or cover and refrigerate up to 12 hours for an overnight bake).

Step 5: Add cheese and bake

Sprinkle the reserved 2 oz (55 g) Gruyère over the top. Bake at 350°F (175°C) for 35 to 40 minutes, until the center is just set, the edges are puffed, and the top is deeply golden with melted, bubbly cheese. If the top browns too quickly, tent loosely with foil for the final 10 minutes.

Step 6: Rest, garnish, and serve

Transfer the dish to a rack and let stand 10 minutes to set the custard. Garnish with chopped chives or parsley. Serve warm, scooping generous squares with plenty of golden edges and melty centers.

Pro Tips

• Day-old croissants absorb custard best; if yours are very soft, toast pieces on a sheet pan at 300°F (150°C) for 8 to 10 minutes to dry them slightly.
• Rinse leeks thoroughly: slice first, then swish in a bowl of cool water to remove grit; lift out and drain well.
• Balance the salt: ham and Gruyère are salty. Start with 1 tsp salt in the custard; add a pinch more only if your ham is very mild.
• For extra browning, broil on high for 1 to 2 minutes at the end, watching closely.
• Let it rest: a 10-minute rest ensures clean slices and a silky custard.

Variations

• Mushroom-spinach: Sauté 8 oz (225 g) sliced cremini with the leeks and fold in 2 cups baby spinach until wilted. Omit ham for a vegetarian version.
• Smoked ham and Swiss: Swap Gruyère for Swiss and use smoked ham; add 1 tsp whole-grain mustard for texture.
• Herb-forward: Replace thyme with a mix of chives and tarragon; finish with lemon zest for brightness.

Storage & Make-Ahead

Assemble up to 12 hours ahead: cover and refrigerate overnight. Bake straight from the fridge, adding 5 to 10 minutes to the cook time. Leftovers keep covered in the refrigerator for up to 4 days. Reheat individual portions in a 325°F (165°C) oven for 12 to 15 minutes, or microwave gently until warmed through. The baked casserole can be cooled, wrapped, and frozen for up to 2 months; thaw in the refrigerator overnight and reheat at 325°F (165°C) until hot.

Nutrition (per serving)

Approximate values: 540 calories; 35 g fat; 31 g carbohydrates; 28 g protein; 1 g fiber; 980 mg sodium. Estimates will vary based on brands and exact ingredients.