In a lg bowl, combine the first 8 ingred. Cover and refrigerate for at least 30 minutes.
For the mayonnaise spread in a small bowl combine the mayonnaise, mustard,(* I used Safeway select country dijon mustard*), onion, garlic and oregano. Set aside.
Grill bread slices uncovered over medium heat 1-2 minutes or until lightly toasted.
Spread mayonnaise mix over toasted side. Grill an additional 1-2 minutes longer until bottom is toasted. Drain tomato mix and spoon over top.