Preheat The Oven'S Broiler. In A Medium Bowl, Toss Together The Tomatoes, Onion, Red Bell Pepper, And Green Bell Pepper. Place The Slices Of Bread On A Baking Sheet, And Place Under The Broiler. Broil For About 2 Minutes, Just Until Lightly Toasted. Remove From The Oven, And Turn The Bread Untoasted Side Up. Place A Handful Of The Vegetable Mixture On Top Of Each Slice. Drizzle With A Bit Of Olive Oil, Then Top With A Slice Of Cheese. Return The Bread Slices To The Broiler, And Cook Until The Cheese Is Melted. Serve Immediately.