Prepare the skillet by preheating a non-stick skillet over medium heat for about 2-3 minutes.

Butter one side of each slice of sourdough bread generously and flip them so the buttered side is down.

Layer 3-4 slices of mozzarella cheese on one slice, add a tablespoon of basil pesto, sun-dried tomatoes if using, and another layer of mozzarella.

Press down gently on the assembled sandwich to keep everything in place.

Butter the top side of the sandwich before placing it in the skillet.

Grill the sandwich on medium heat for about 3-4 minutes, press down gently to help the cheese melt.

Flip the sandwich and cook for an additional 3-4 minutes until golden brown and the cheese is melted.

Remove from the skillet, let cool for a minute, then slice in half and serve.