1. Spread one slide of bread slice with butter and place butter-side down in skillet on medium heat.
2. Spread pesto on the =side of the bread facing up in the skillet and top with 1 slice of mozzarella and 2 tomatoes.
3. Spread pesto onto 1 side of another slice of bread, and spread butter on the other side. Place on sandwich butter side up.
4. Heat sandwich, flipping half way through, until both sides are golden brown and cheese is melted (about 5 minutes).