Overnight Croissant Breakfast Casserole Recipe

Name: Overnight Croissant Breakfast Casserole Recipe
Ingredients: 1 tablespoon olive oil; 1 small onion, peeled and diced; 1-pound roll of breakfast sausage (mild flavor); 4 large buttery croissants or 8 small croissants, torn into large bite-sized pieces; 1 cup shredded cheddar cheese; 6 large eggs; ½ cup heavy cream; 3 cups milk (whole milk recommended), divided (Only 1/2 cup of you don't make the gravy); ½ teaspoon dry mustard powder; ½ teaspoon salt (plus additional to taste for the gravy); ½ teaspoon black pepper (plus additional to taste for the gravy); ¼ teaspoon paprika; ¼ teaspoon garlic powder; 2 tablespoons butter; 3 tablespoons all-purpose flour