Make the Mornay sauce: Prepare a béchamel by melting the butter in a saucepan, then add the flour. Stir and cook for a few minutes without letting it brown, then set aside until completely cooled. Heat the milk and pour it over the flour mixture while stirring. Season with salt, pepper, and nutmeg. Return to low heat and cook for 3 minutes, stirring constantly until thickened.

Mix the egg yolk with the sour cream, then add to the cooled béchamel at room temperature. Heat gently for 3 minutes, stirring, then add the shredded Swiss cheese. Stir until the cheese is fully melted and the sauce is smooth.

If you’re not using the Mornay sauce right away, dot the surface with small pieces of butter to prevent a skin from forming.

Preheat the oven to 350°F (180°C). Slice each croissant in half horizontally, but don’t cut all the way through.

Spoon a generous tablespoon of Mornay sauce inside each croissant, then add a slice of ham.

Top each croissant with another spoonful of Mornay sauce and sprinkle with shredded Swiss cheese. Bake until the top is golden and the inside is hot. Serve with a salad if you like.