1. Cut a thin slice from stem end of each tomato. Scoop out and discard pulp, reserving shells; set aside.
2. In small bowl combine remaining ingredients, mixing well. Fill each reserved tomato shell with an equal amount of scallion-cheese mixture.
3. Preheat broiler. Set tomato shells on nonstick baking sheet and broil until cheese is bubbly, about 1 minute. Serve warm.