Overnight Croissant Breakfast Casserole

Name: Overnight Croissant Breakfast Casserole
Ingredients: 1 tablespoon olive oil; 1 small onion (peeled and diced); 1- pound breakfast sausage (mild flavor); 4 large croissants (or 8 small croissants, torn into large bite-size pieces); 1 cup shredded cheddar cheese; 6 large eggs; ½ cup heavy whipping cream; 3 cups milk (whole milk recommended, divided); 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