Preheat the oven to 350 degrees F (175 degrees C).
Heat a large skillet over medium-high heat. Cook and stir turkey sausage in the hot skillet until browned and crumbly, 5 to 7 minutes.
Spray a 9-inch square baking dish with cooking spray and line with crescent rolls; place the triangles flat in the dish so they cover the whole bottom.
Bake in the preheated oven until just golden, 10 to 12 minutes.
Meanwhile, beat eggs, egg whites, milk, baking mix, mustard, salt, and pepper together in a bowl before adding Cheddar cheese.
Remove pan from the oven and let cool at least 5 minutes. Spread the browned sausage over the rolls and pour egg mixture over it al. Cover and refrigerate the unbaked casserole at least 8 hours to overnight.
In the morning, preheat the oven to 350 degrees F (175 degrees C).
Bake the casserole in the preheated oven until set, 40 to 60 minutes. Let cool before serving.
