Feel free to swap in chopped broccoli for the asparagus.
Coat a large non-stick skillet with cooking spray. Crumble the sausage into the skillet and cook, breaking up large pieces and stirring until the meat is cooked through, about 7 minutes. Transfer to a plate to cool.
Whisk the egg whites, eggs, and milk in the insert of a 6-qt. slow cooker. Stir in 1 cup of cheddar, salt, and pepper. Stir in bread, onion, asparagus, and the browned sausage. Layer the remaining cheese over top. Cover and cook on low for 2 hours. Let cool uncovered in the slow cooker for about 20 minutes before serving.