Preheat oven to 400°. In a small skillet, heat butter over medium heat. Add onion and cook until tender. Remove from the heat. Stir in the cheese, sour cream, mayonnaise and seasonings; set aside. , In a bowl, combine the baking mix and milk to form a soft dough. Turn dough onto a well-floured surface; knead lightly 10-12 times. Pat into a greased 13x9-in. baking dish, pushing dough up sides of dish to form a shallow rim. Arrange tomato slices over top. Spread with topping; sprinkle with paprika. , Bake for 25 minutes. Let stand for 10 minutes before cutting.