Preheat the oven to 375°F. Cut the croissants into 1-inch cubes, or just tear apart with your hands. Spread on a sheet pan lined with parchment paper and toast for about 8-10 minutes. Remove from the oven and cool; turn off the oven.
2 packages mini croissants* (14 oz.)
Meanwhile, cook the sausage in a saute pan over medium-high heat breaking it up with a wooden spoon until fully cooked. Drain the sausage in a colander and discard the grease. Pour the cooked sausage into a large mixing bowl.
1 lb. ground pork sausage
To the same saute pan, melt the butter. Add the onions and cook, stirring occasionally, until fragrant, about 5 minutes. Add the garlic, stir for another 30 seconds or so. Transfer to the bowl with the sausage. Add the toasted croissants to the bowl, along with the Gruyere cheese. Stir until combined.
1 Tbsp. unsalted butter, 1/2 white onion, diced small, 3 cloves garlic, minced, 1 cup Gruyere cheese, shredded
Grease a 9x13 inch casserole dish with baking spray and pour the sausage croissant mixture into the dish in an even layer.
In another mixing bowl whisk together the eggs, whole milk, heavy cream, Dijon mustard and heavy pinch of salt and pepper. Whisk until combined.
8 large eggs, 1 1/2 cups whole milk, 1 cup heavy cream, 1 Tbsp. Dijon, salt and pepper
Pour the egg custard mixture over the croissants. Cover with plastic wrap and refrigerate for at least 1-2 hours, or up to 8 hours/overnight*
When ready to bake, preheat the oven to 375°F.
Sprinkle the mozzarella cheese on top. Bake uncovered for 35-40 minutes or until the tops are golden brown and the casserole is set. (Check around 30 minutes to make sure the tops aren't too browned, if they are cover with foil for the last few minutes).
1 cup mozzarella cheese, shredded
Let the casserole rest for about 10 minutes before slicing. Garnish with chives, parsley, your favorite hot sauce or black pepper. Enjoy!