Assemble sandwich by spreading pesto onto one piece of both pieces of bread.
Layer mozzarella and sliced tomatoes on one slice of bread. Sprinkle tomato with salt and pepper and top with second piece of bread.
Melt butter in a skillet over medium to medium-high heat. Swirl butter around to coat the bottom on pan and toast sandwich on both sides, adding more butter, if needed.
Serve warm.