In a bowl, combine oil, basil, garlic, salt and pepper.  Add tomatoes and onions, toss gently. Sprinkle with cheese. Refrigerate for at least 1 hour. Bring to room temperature before serving. Cut bread into 24 slices; toast under broiler until lightly browned (you can brush the bread with olive oil or butter prior to browning if desired). Top with tomato mixture.  Serve immediately.