01 -
Heat olive oil in a skillet. Add chopped onions and sauté until soft and translucent. Add sausage and cook until browned, draining the fat. Reserve a portion of the fat for the gravy. Allow the mixture to cool slightly.
02 -
Combine torn croissants, cooked sausage mixture, and shredded cheddar cheese in a bowl. Mix thoroughly and spread into a greased 9×13-inch baking dish.
03 -
In a bowl, whisk together eggs, heavy cream, 1/2 cup of milk, mustard powder, salt, black pepper, paprika, and garlic powder. Pour this mixture evenly over the croissant and sausage mixture in the baking dish. Cover with plastic wrap and refrigerate overnight.
04 -
Preheat oven to 350°F (175°C). Remove the casserole from the refrigerator and bake for 35 to 45 minutes, until the top is golden brown.
05 -
In a saucepan, melt butter along with the reserved sausage fat. Stir in all-purpose flour and cook for 1 minute, stirring constantly. Gradually whisk in 2 1/2 cups of milk and cook until thickened. Season with salt and pepper to taste.
06 -
Serve the baked casserole hot with the prepared gravy on the side.