Heat oven to 400. Spray a 9 inch pie pan or 13x9 pan with cooking spray. In skillet, cook turkey and onion over med. high heat 5 to 7 min. stirring occasionally until turkey is no longer pink. Stir in taco seasoning mix, spread in pan. In med. bowl stir bisquick mix, milk and eggs with a wire whisk or fork until blended. Pour into pan over turkey mixture. Bake 25 min. Top with cheese and tomato. Bake 2 to 3 min. longer or until cheese is melted. Let stand 5 min.