Heat oven to 350 degrees. Core the tomatoes, cut crosswise into ¾-inch slices. Place in single layer on rack in broiler pan. Mix mayonnaise, 1 tablespoon cheese, basil and mustard. Spread about 1 teaspoon over each tomato slice. Sprinkle 1 tablespoon cheese over tomatoes with tops 6 inches from heat 2 to 4 minutes until topping is golden and bubbly.