Overnight Croissant Breakfast Casserole

Name: Overnight Croissant Breakfast Casserole
Ingredients: 1 tbsp olive oil; 1 small onion (peeled and chopped); 1 lb breakfast sausage (mild flavor); 4 large butter croissants or 8 small croissants; 1 c shredded cheddar cheese; 6 large eggs; 1/2 c heavy whipping cream; 3 c milk (whole milk recommended, divided); 1/2 tsp dry mustard powder; 1/2 tsp salt (plus additional for the gravy to taste); 1/2 tsp black pepper (plus additional for the gravy to taste); 1/4 tsp paprika; 1/4 tsp garlic powder; 2 tbsp butter; 3 tbsp all-purpose flour