01 -
Start with a cold skillet, add the bacon and turn heat to medium. Cook until crispy, then transfer to a paper towel-lined plate. Keep the bacon grease in the pan for cooking the sandwiches.
02 -
Spread 1/2 tablespoon of pesto on one side of each bread slice. On two slices, layer the mozzarella, tomatoes, and bacon (if using). Top with the remaining bread slices, pesto side down.
03 -
Heat the skillet with bacon grease (or start with a clean pan) over medium heat. Butter the outsides of the sandwiches and place them in the hot pan, butter side down. Cook until golden brown, about 3-4 minutes.
04 -
Spread remaining butter on the tops, then flip the sandwiches. Cook another 2-3 minutes until bread is toasted and cheese is melted. Let cool slightly before cutting and serving.