Generously grease the inside of a slow cooker with cooking spray. Set aside 1 tablespoon each of Parmesan and Cheddar cheeses.
Combine baking mix, milk, remaining Parmesan and Cheddar cheeses, egg whites, minced onion, and garlic salt in a bowl until well mixed. Pour into the prepared slow cooker and sprinkle reserved Parmesan and Cheddar over top.
Cook on High until bread is cooked through, about 1 hour.