01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking dish with unsalted butter or nonstick cooking spray.
02 - Distribute torn croissant pieces evenly in the buttered baking dish.
03 - In a large mixing bowl, whisk together eggs, whole milk, heavy cream, minced garlic, chopped chives or parsley, salt, ground black pepper, and dried thyme until smooth and well-combined.
04 - Stir in shredded cheddar cheese, shredded mozzarella cheese, and half of the grated Parmesan until incorporated.
05 - Pour the egg and cheese mixture evenly over croissant pieces, pressing down gently to fully saturate all bread.
06 - Drizzle melted unsalted butter over the surface. Sprinkle remaining grated Parmesan cheese evenly over the top.
07 - Allow the mixture to rest for 10 minutes so croissants absorb liquid.
08 - Transfer casserole to oven and bake for 30 to 35 minutes, until golden brown and center is set.
09 - Let casserole cool for 5 minutes. Garnish with additional chopped chives or parsley before serving.