Combine Bisquick mix with milk and 2 Tbsp mayonnaise. Stir and spread into greased 13 x 9 pan. Sauté sausage and onion in oil, spread over Bisquick mix. Beat egg with remaining mayonnaise, cheese and chilies (optional). Spread over meat. Bake at 375 ºF for 25 minutes. Cut into one inch squares and serve.