In A Large Bowl, Combine The First Eight Ingredients. Cover And Refrigerate For At Least 30 Minutes. For Mayonnaise Spread, In A Small Bowl, Combine The Mayonnaise, Mustard, Onion, Garlic And Oregano; Set Aside. Grill Bread Slices, Uncovered, Over Medium-Low Heat For 1-2 Minutes Or Until Lightly Toasted. Turn Bread; Spread With Mayonnaise Mixture. Grill 1-2 Minutes Longer Or Until Bottoms Of Bread Is Toasted. Drain Tomato Mixture; Spoon Over Tops.