In A Nonstick Skillet, Cook Beef Over Medium Heat Until No Longer Pink; Drain. Remove From The Heat; Stir In The Cheese, Mayonnaise, Sour Cream And Onion. In A Bowl, Combine Biscuit Mix And Water. Spread Into An 8-In. Square Baking Dish Coated With Nonstick Cooking Spray. Top With Beef Mixture And Tomato. Bake, Uncovered