Grease a 9x13 casserole or baking dish with 1 tablespoon of unsalted butter.
Cut croissants into half lengthwise (like you'd do for a sandwich). Place croissant halves in a single layer, evenly spaced until you cover the base of the baking dish. Set aside the remaining croissant halves while making the custard.
In a medium-sized bowl: whisk 8 large eggs along with 3 tablespoons of brown sugar, 1 tablespoon of vanilla extract, 0.5 teaspoons of cinnamon, nutmeg, and ¼ teaspoon of kosher salt until they are well combined. Then, whisk in 1 cup each of heavy cream and milk until it's a smooth custardy mixture.
Pour half this custard mixture evenly over the croissant halves in the baking dish. Use a flat spatula to push the croissant into the custard so it soaks it up. Then, place the remaining croissant halves on another even layer on top. Pour the remaining custard on top. Cover with aluminum foil. Ideally, you want the croissant to soak up the custard for at least 30 minutes in the fridge, but I often make this the night before and leave it in the fridge overnight. If you leave it overnight, take it out 30 minutes before you'll bake it.
Set the oven rack to the center position and preheat the oven to 375°F.
Bake in the oven for about 45-50 minutes. If it starts to brown too quickly, you can make a loose tent with aluminum foil.
Cool for a few minutes minutes and top with maple syrup, powdered sugar, berries, nuts or toppings of choice before serving.
This is great when made ahead of time and chilling it overnight really elevates the dish to a whole new level! Once cooked though, it can be reheated in an oven, air fryer or microwave oven, but I don't recommend freezing it.
If you want a nice crumble topping: in a small bowl, combine ½ cup of all purpose flour, ½ cup of brown sugar, and ½ cup of unsalted butter along with ½ teaspoon cinnamon and ¼ teaspoon kosher salt. Use a fork to mash up the butter so it's all evenly mixed. You can also use a hand mixer if you're feeling adventurous! Sprinkle this on the croissants right before you bake. Delicious!