Pierce potatoes with fork. Microwave on HIGH 10 minutes or until cooked through, turning over after 6 minutes.
Cook beef and onion in large skillet over medium-high heat 7 minutes or until beef is crumbled and no longer pink, stirring occasionally; drain. Add chili and beans; heat until hot.
Divide salad greens evenly between 4 dinner plates. Place 1 cooked potato on greens; split tops of each potato in crisscross pattern and press ends to open widely. Evenly divide chili mixture, cheese and drained tomatoes over each potato. Top salad with dressing, if desired.