Slice tomatoes in medium slices (should have about 12 slices for 4 sandwiches, or 3 slices per sandwich), and place them in a shallow dish. Pour 2 TBS balsamic vinegar over tomatoes. Let sit while preparing the rest.

Instructions to make 1 sandwich, repeat until you have desired number of sandwiches (recipe is for 4):

Butter one side of the slices of bread lightly. Turn over, so butter is facing down. Top 1 slice of bread with 1 TBS of pesto, add 1 slice of cheese, 3 tomatoes, and then another slice of cheese. If you like it really pesto-y, add some more pesto to top slice. Place top bread slice butter side out.

Heat skillet or panini press, and put sandwich on to cook. If using a skillet, flip when golden brown on one side. I cooked on a skillet for about 3 minutes on medium on each side. I cooked 2 sandwiches at a time and was easily able to flip with enough room in the pan. Repeat until all sandwiches are cooked.

Cut in half with bread knife and enjoy!