When I lived in Busan, Korea, there was a great cafe called NoNaim Cafe which was extremely popular with the expatriate crowd. I used to go there every weekend to have one of these sandwiches, and when I left Korea, I figured out how to make it myself. I have never had a boyfriend who was not wowed by this sandwich.
Spread pesto on both slices of bread. Arrange tomatoes over pesto, sprinkle with salt and pepper (if desired), arrange mozzarella slices over pesto, close sandwich, and grill both sides until the outside is gold and crispy and the mozzarella is just started to melt. You can also add some spinach leaves to this sandwich, as well, although I almost never have them on hand. Can serve with olive oil and balsamic vinegar for dipping.