Preheat oven to 375℉.
In a large skillet, cook sausage over medium heat until no longer pink, breaking into crumbles, 5 to 7 minutes; drain and set aside. To sausage, add cut asparagus, diced onion, salt and pepper to taste. Stir to combine.
Grease a 9×13 inch baking dish with cooking spray or butter. Place bagel pieces into the bottom of the dish. Top with sausage, asparagus and onion mixture. Then sprinkle with 1½ cups cheese.
In a large bowl, whisk together 6 whole eggs, 6 egg whites, 2 cups milk, salt and pepper to taste. Pour over layers. Bake immediately or refrigerate, covered, overnight. Note: If refrigerated, remove casserole while oven preheats.
Bake, covered, 30 minutes, then uncovered, 15 minutes or until a knife inserted near the center comes out clean. Let stand 5 to 10 minutes before cutting to serve.