Croissant Breakfast Casserole With Ham and Gruyère

• Croissants create a lighter top than sandwich bread, but their buttery layers still hold enough custard for a substantial square. Tearing them rather than slicing exposes more edges, so the mixture settles evenly through the dish.
• Leeks bring a gentle onion note that stays in the background, while diced ham adds salt and substance without requiring a separate sauce at breakfast.
• Using Gruyère in two layers gives the center a creamy pull and leaves browned pockets across the surface, where the cheese meets exposed pastry.

• 6 day-old croissants, torn (360 g)
• 1 tablespoon butter (14 g)
• 2 leeks, sliced (250 g)
• 1 1/2 cups cooked ham, diced (225 g)
• 2 cups Gruyère, divided (200 g)
• 10 large eggs (500 g)
• 1 cup whole milk (240 g)
• 1 cup half-and-half (240 g)
• 1 tablespoon Dijon (15 g)
• 1 teaspoon kosher salt (6 g)
• 1/2 teaspoon black pepper (1 g)
• 1/4 teaspoon nutmeg (1 g)
• 1 teaspoon fresh thyme, chopped (1 g)

Ingredient notes

• Use plain, day-old butter croissants, not filled or iced ones. Their slightly dry interior absorbs the custard while the flaky exterior leaves distinct, browned ridges on the top of the casserole after baking.
• Choose thick-cut cooked ham and dice it small, so every portion gets some without large chewy pieces. Leftover roasted ham works too; blot visibly wet pieces before adding them to the dish evenly.
• Leeks need a thorough rinse between their layers, where grit often hides. Cook them until soft, not browned, because their moisture and mild flavor should blend into the custard rather than dominate it here.
• Whole milk and half-and-half make a custard that is rich without becoming heavy. Do not swap all the dairy for cream; the croissants already contribute plenty of butter and need enough liquid to soak through.
• Whisk the eggs completely before adding dairy, which prevents stubborn strands of white in the finished casserole. Dijon is a background seasoning here; choose smooth Dijon rather than a coarse, strongly seeded mustard for this.

1. 1Heat the oven

   Set an oven rack in the middle position and heat the oven to 350°F. Butter a 9-by-13-inch baking dish, including the corners, then place it on a rimmed sheet pan. The sheet pan catches any custard that bubbles over and makes the full dish easier to move.

   

2. 2Soften the leeks

   Melt butter in a skillet over medium-low heat. Add the rinsed leeks with a pinch of salt and cook, stirring often, until soft and glossy, about 7 minutes. Scrape them onto a plate and let them cool slightly so they do not begin cooking the eggs when combined.

   

3. 3Tear the croissants

   Pull the croissants into rough, bite-size pieces rather than neat cubes. Scatter half in the buttered dish, leaving some flaky pieces facing upward. Sprinkle over the ham, cooked leeks, and half the Gruyère, then add the remaining croissant pieces without packing them down.

4. 4Whisk the custard

   In a large bowl, whisk the eggs until fully blended. Whisk in the milk, half-and-half, Dijon, thyme, salt, pepper, and nutmeg. Pour slowly over the croissants, moving around the dish so dry corners receive liquid. Press only the submerged pieces gently with a spatula.

5. 5Rest and top

   Let the unbaked casserole stand for 20 minutes at room temperature. This gives the croissant interiors time to drink in the custard while the upper ridges remain exposed. Sprinkle the remaining Gruyère across the top just before baking for more defined browned cheese patches.

6. 6Bake and settle

   Bake until puffed, browned at the edges, and 160°F in the center, about 40 to 45 minutes. Rest the casserole for 10 minutes before cutting. The custard finishes settling during that pause, giving portions cleaner edges and keeping the flaky top from collapsing under the knife.

• Keep the croissant pieces uneven. Small pieces absorb custard quickly, while larger flakes create the craggy top that distinguishes this from a bread pudding.
• Rinse sliced leeks in a bowl of water, lift them out with your hands, and leave any grit behind. A colander alone can leave particles trapped between layers.
• If the top browns before the center reaches 160°F, lay foil loosely over the dish. Do not seal it tightly, or trapped steam will soften the exposed croissant edges.
• Let the baked casserole stand for the full 10 minutes. Cutting immediately releases more custard and gives less tidy squares for containers.

Cut the casserole into twelve squares and serve with crisp apple slices, grapes, or a citrus salad that offsets the buttery pastry. A spoonful of Dijon mixed with sour cream makes a quick table sauce for anyone who wants more tang. For a larger brunch spread, pair it with simple roasted potatoes and coffee. The portions are substantial enough that fruit is usually the only extra needed for a weekday plate.