Heat oven to 350°F. In large bowl, mix beef, onion, bread crumbs, sage, salt and egg until well blended. Press in bottom of ungreased 8-inch square (2-quart) glass baking dish. Spread mashed potatoes evenly over top. Sprinkle evenly with cheese.
Bake uncovered about 50 minutes or until meat thermometer inserted in center of beef mixture reads 160°F. Sprinkle with tomato, bacon and green onions.