Nancy Silverton's Easy Twice-Baked Ham and Cheese Croissants

Make the béchamel. Set a fine-mesh sieve over a bowl.

Put the onion slices, butter, bay leaf and chile in a medium saucepan and cook over medium-low heat until the onion is tender and translucent, stirring often so it doesn’t brown, 10 to 12 minutes. Add the flour and cook off the flour flavor, stirring constantly with a whisk, about 3 minutes. Gradually add the milk, whisking constantly.

Increase the heat to medium-high and bring the sauce to a boil, stirring constantly with the whisk. Reduce the heat to low and simmer, stirring constantly until the sauce is thick enough to coat the back of a spoon, 3 to 4 minutes. Remove from the heat and strain the sauce through the fine-mesh sieve into the bowl, discarding the contents of the sieve.

Stir in the salt and nutmeg and set aside to come to room temperature. Once the sauce is at room temperature, cover the bowl with plastic wrap and refrigerate until the sauce is chilled, at least 30 minutes.

Assemble and bake the croissants. Place a baking rack in the center of the oven and preheat the oven to 375F.

Using a serrated knife, cut a 4-inch long slit along the top of each croissant, as you would cut open a baked potato, stopping short of cutting all the way through to the bottom of the croissant. Splay open the croissants.

Spoon 2 tablespoons of béchamel inside each sliced croissant. Use the back of the spoon to spread the sauce to the edges, covering the inside of the croissants in a thin layer and pushing the béchamel into the croissants so they absorb the sauce. Sprinkle 1/4 cup of the grated Gruyère over the sauce in each croissant.

Tear each prosciutto slice into two pieces and lay 4 pieces (from 2 slices) inside each croissant, with some of the prosciutto flopping over the top edges of the sliced croissant. Push the halves of the croissant together to close them. Spoon 1 tablespoon of béchamel on top of each croissant and use the back of the spoon to spread the sauce over the surface. Sprinkle 1/4 cup of Gruyère over each croissant, pressing it into the béchamel so the cheese adheres. Sprinkle the thyme over the croissants, dividing it evenly. Grind a few turns of pepper over each croissant. Cover a baking sheet with parchment paper and place the croissants on top, spacing them evenly.

Bake the croissants until the cheese on top is golden brown, 18 to 25 minutes, rotating the baking sheet from front to back halfway through so the croissants bake evenly. Remove the baking sheet from the oven and use a fine Microplane or fine grater to grate parmesan over the top of the croissants.

The croissants are best eaten warm. Serve them fresh from the oven or place them in a 350F oven for about 10 minutes to reheat them.