In a skillet, cook beef over medium heat until no longer pink; drain. Stir int he chili powder, salt and pepper. Remove from the heat and set aside.
Combine biscuit mix and water to form a soft dough. Press onto the bottom and up the sides of two 10-oz. ramekins or custard cups coated with nonstick cooking spray. Fill with meat mixture; top with tomato and green pepper. Combine the sour cream, mayonnaise, cheddar cheese, and onion. Bake, uncovered at 375 for 20-25 minutes or until heated through.