Heat oven to 400°F Spray a 8 or 9-inch pan with pan spray. Stir together Bisquick mix, water & egg. Spread in the pan then sprinkle with 1 1/4 cups of the cheese.
Using a skillet cook & stir chicken in oil over medium-high heat until outsides turn white; drain. Stir in salsa & chopped onion; heat until hot. Spoon over batter in the pan to within 1/2-inch of the edges.
Bake 22-25 minutes or until edges are dark golden brown. Sprinkle with remaining cheese. Bake 1-3 minutes or until cheese melts; loosen from sides of the pan & serve.