1. Cut out stem end from each tomato. If necessary, from bottom of each tomato, cut thin slice so that tomato will sit level. Cut each tomato horizontally in half. Arrange halves, cut side up, around edge of 10-inch pie plate or large dinner plate; set aside.
2. In medium-sized bowl, place butter or margarine; cover with large plate. Cook at high (100% power) 45 seconds or until butter or margarine melts. Squeeze spinach to remove as much moisture as possible. Stir spinach and crumbled cheese into butter or margarine. Spoon spinach mixture onto tomato halves; cover tomatoes with waxed paper. Cook at high 10 to 12 minutes until spinach mixture is hot, rotating tomatoes in pie plate a half turn after 6 minutes.