Heat oven to 350. Spray 11x7 or any glass baking dish larger than 9x9. Spray dish with cooking spray. In a large bowl, beat egg and milk with wire wish until smooth. Cut biscuits into quarters gently stir into egg mixture to coat.
Fold in sausage, cheese and onion. Spoon mixture into pan arranging in single layer, and biscuits touching.
Bake at 350 for 25 minutes or until golden brown. Cut into squares.