In a small bowl, combine the biscuit mix, milk and 2 tablespoons mayonnaise. Spread into a greased 11x7-in. baking dish.
In a large skillet, cook the sausage and onion over medium heat until meat is no longer pink; drain. Spoon over biscuit mixture. In a large bowl, combine the egg, chilies and remaining mayonnaise; spread over sausage mixture. Sprinkle with cheese.
Bake, uncovered, at 350° for 30-35 minutes or until golden brown.