In a large skillet, brown the sausage over medium heat until fully cooked; transfer to a paper towel-lined plate to remove grease, then place in a medium mixing bowl. Wipe the pan clean, then sauté the onion until translucent, add spinach, and wilt. Add the onion and spinach to the bowl with the sausage, then toss with the cheese and cubed croissants, spreading the mixture into an 8x8-inch baking pan.

In a separate medium mixing bowl, whisk together the eggs, then whisk in the milk, half and half, dry mustard, salt, black pepper, paprika, and garlic powder. Pour this egg mixture over the ingredients in the baking pan, ensuring all croissant pieces are well-soaked. Cover the pan and refrigerate for at least 8 hours, preferably overnight.

Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Bake the uncovered casserole for 45-50 minutes, or until the center is set and the top is golden brown. Remove from the oven and allow it to rest for 10 minutes before serving.