I love the combination of tomato and cheese. The short marination of the tomatoes in the olive oil and the balsamic vinegar gives it a nice kick. Other vinegar is not as nice. If I have it in the fridge I add an extra topping of mozzarella and you can please the meat eaters by adding slices of crunchy bacon. You can make it a closed sandwich by just toasting 4 more slices of bread to top the sandwich.