Preheat a non-stick skillet or griddle over medium heat for about 3 minutes.

Spread approximately 1 tablespoon of pesto on one side of each slice of bread.

On two slices of the pesto-covered bread, place a slice of mozzarella cheese, then layer two slices of ripe tomatoes.

Add another slice of mozzarella on top of the tomatoes and close the sandwich with the remaining slices of bread, pesto-side down.

Spread approximately 2 tablespoons of butter on the outer sides of the sandwich.

Place the sandwich in the preheated skillet, butter-side down, and grill for 3–4 minutes until golden brown.

Carefully flip the sandwich to grill the other side for an additional 3–4 minutes until golden and cheese is melted.

Let the sandwich rest on a cutting board for about 1 minute before slicing.

Slice the sandwich in half and serve immediately.