Grease or coat a 6 quart slow cooker with cooking spray.Lightly beat together the eggs, milk, dry mustard, salt, and pepper.Place about a third of the shredded cauliflower in an even layer in the bottom of the slow cooker, and top with about a third of the onion. Season with salt and pepper, the top with about a third of the sausage and a third of the cheese. Repeat the layers two more times.Pour the egg mixture over the contents of the slow cooker.Cook on low for 5-7 hours, or until eggs are set and the top is browned.