Slice the croissants in half and place the bottom halves in the casserole dish. Set the top of the croissants to the side.
Heat a skillet over medium to medium low heat. Once the pan has heated up melt a tablespoon of butter and add the leeks with a pinch of salt to sweat them. You don't want them to brown, so adjust the heat as needed and occasionally stir for about 5-7 minutes until softened.
Fill the croissants: Spoon the leeks over the croissants in the casserole dish distributing evenly. Then arrange the ham on top of the leeks.
In a medium bowl thoroughly whisk together the eggs, egg yolk, half & half, creme fraiche, pecorino romano and nutmeg.
Pour the custard into the casserole dish and arrange the croissant tops, lightly pressing them down into the custard.
Cover with plastic wrap and place in the refrigerator for 1 hour or overnight.
Preheat the oven to 350
Sprinkle the gruyere cheese all over the top of the casserole and bake uncovered for about 45 min. If it begins to brown too much, tent it with aluminum foil and continue to bake until the center is cooked through.