Overnight Croissant Breakfast Casserole with Sawmill Gravy

Name: Overnight Croissant Breakfast Casserole with Sawmill Gravy
Ingredients: 1 TBSP olive oil; ½ cup finely chopped onion; 1 lb. roll of breakfast sausage; 4 large croissants or 8 small croissants, torn or chopped into 1-inch pieces; ½ cup Monterey jack cheese; ½ cup grated cheddar cheese; 6 large eggs; ½ cup heavy whipping cream; 1½ cups whole milk (divided); ½ tsp. dry mustard powder; ¼ tsp. paprika; ¼ tsp. garlic powder; ½ tsp. salt (plus additional to taste); ½ tsp. ground black pepper (plus additional to taste); 1 TBSP butter; 2 TBSP all-purpose flour