Open-face sandwiches make an easy lunch for a crowd.
Preheat broiler. Arrange tomatoes in a single layer on a rimmed baking sheet. Drizzle with 3 tablespoons oil, and season with salt and pepper. Broil until tomatoes are soft, about 5 minutes.
Melt butter with remaining tablespoon oil. Brush over cut sides of bread. Broil until top edges are just golden.
Arrange tomatoes over bread, slightly overlapping. Top with cheese. Place on baking sheet; broil until cheese has melted and is bubbling, 1 to 2 minutes.