Prepare the turkey bacon: If you are using turkey bacon, start by adding it to a cold skillet. Turn the heat to medium and cook until crispy, which usually takes about 5-7 minutes. Once done, transfer the bacon to a paper towel-lined plate to drain, keeping the grease in the pan for extra flavor.
Assemble the sandwiches: On one side of each slice of bread, spread 1/2 tablespoon of basil pesto. On two of the slices, layer the sliced mozzarella, followed by the thinly sliced tomatoes, and the crispy turkey bacon if you chose to include it. Top each with the remaining slices of bread, pesto side down.
Heat the skillet: Warm the skillet with the turkey bacon grease over medium heat. If you didn't use turkey bacon, just apply a bit of olive oil or use fresh butter. Spread 1/2 tablespoon of softened butter on the top side of each sandwich.
Cook the sandwiches: Carefully place the sandwiches in the pan, butter side down. Cook them for about 3-4 minutes or until the bottom is golden brown and crispy. Once golden, spread the remaining butter on the top sides of the sandwiches, flip them over, and cook for another 2-3 minutes until the other side is also golden, and the cheese has melted.
Cool and serve: Let the sandwiches cool for 1-2 minutes to prevent burns, then cut them in half to enjoy your delicious creation!