Sausage Mixture:

• 1 tbsp olive oil
• 1 small onion, peeled and chopped
• 1 lb breakfast sausage, mild flavor

Casserole:

• 4 large butter croissants or 8 small croissants
• 1 cup shredded cheddar cheese
• 6 large eggs
• 1/2 cup heavy whipping cream
• 3 cups 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

Gravy:

• 2 tbsp butter
• 3 tbsp all-purpose flour