Gather your ingredients, ensuring butter (or mayonnaise) is at room temperature for easy spreading.
Spread 1 tablespoon of pesto evenly on one side of each slice of bread.
Place a slice of provolone cheese on one pesto-covered slice.
Assemble the sandwich with the other slice pesto side down on top of the cheese.
Spread 1 tablespoon of butter (or mayonnaise) on the outer side of the sandwich.
Heat the skillet over medium heat for even toasting.
Place the sandwich in the skillet, buttered side down, and cook for about 3-4 minutes until golden brown.
Flip the sandwich using a spatula, adding more butter (or mayonnaise) if desired, and cook for another 3-4 minutes.
Remove from skillet, let cool for a minute, then slice and serve.