Heat olive oil in a large skillet over medium heat. Add chopped onions and sauté until soft and translucent, about 3-4 minutes.

Add breakfast sausage to the skillet and cook until completely browned, breaking it apart as it cooks. Drain the fat but reserve 2 tablespoons for the gravy.

Let the sausage mixture cool for about 10-15 minutes while preparing the croissants.

Tear croissants into bite-sized pieces (about 2-inch chunks) and place in a large bowl with the cooled sausage mixture and shredded cheddar cheese. Toss to combine.

Spread the croissant mixture evenly in a greased 9x13-inch baking dish (use butter or cooking spray).

In another bowl, whisk together eggs, heavy cream, 1/2 cup milk, dry mustard, salt, pepper, paprika, and garlic powder until smooth and well combined.

Pour the egg mixture evenly over the croissant mixture, gently pressing down with a spatula to help absorption. Cover tightly with plastic wrap or lid and refrigerate overnight (at least 8 hours).

The next morning, preheat oven to 350°F (175°C). Remove cover from casserole and bake for 35-45 minutes until the top is golden brown and the center is set (should not jiggle when gently shaken).

While casserole bakes, prepare gravy: Melt 2 tablespoons butter with reserved sausage fat in a saucepan. Whisk in flour and cook for 1 minute, stirring constantly.

Gradually add remaining 2 1/2 cups milk to the gravy while whisking constantly. Continue stirring until gravy thickens to desired consistency. Season with salt and pepper to taste.

Let casserole rest for 10 minutes before serving. Serve hot with homemade sausage gravy on the side.