In a large skillet, brown the sausage over medium heat until fully cooked. Transfer the sausage to a paper towel-lined plate. Remove the grease by slightly patting it with the paper towels, then place it in a medium mixing bowl. Wipe the pan out a bit, then sauté the onion until translucent, add the spinach and wilt. Add to the bowl with the sausage, then toss with the cheese and cubed croissants. Spread into the bottom of an 8x8-inch baking pan.
In a medium mixing bowl, whisk together the eggs, then add and whisk in the milk, half and half, mustard, salt, black pepper, paprika, and garlic powder. Pour over the egg mixture in the pan, taking care to soak the tops of all of the croissant pieces. Cover and refrigerate for at least 8 hours, preferably overnight.
Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Bake uncovered for 45-50 minutes or until the center is set and the tops are golden brown. Remove from the oven and allow to rest for 10 minutes before serving.
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Notes
If this recipe is doubled, then bake it in a 9x13-inch baking dish or pan for 50-60 minutes.To STORE the Croissant Breakfast Casserole leftovers, place them in an airtight container and cover them tightly. You can refrigerate it for up to 3 days. To FREEZE, wrap it well with plastic wrap and aluminum foil, then freeze for up to 3-6 months.To REHEAT, thaw overnight in the fridge if frozen, then bake at 350°F (175°C) for 20-25 minutes, or until heated through. Individual portions can be microwaved for 1-2 minutes until warm.