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 with the paper towels, then place in a medium mixing bowl. Wipe the pan out a bit, then saute 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 a 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 and pepper, paprika, and garlic powder. Pour over the 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. 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.
Video
Notes
If this recipe is doubled, then bake it in a 9x13-inch pan for 50-60 minutes.