Start by softening your unsalted butter at room temperature for easy spreading. Slice fresh mozzarella into thick rounds, quarter ripe tomatoes, and wash the basil leaves.

Butter one side of each sourdough slice generously. On the unbuttered side of one slice, layer creamy mozzarella, quartered tomatoes, and fresh basil. Drizzle balsamic vinegar, season with salt and pepper, and top with the second slice buttered side facing out.

Preheat a skillet over medium heat for 2-3 minutes. Place your assembled sandwich in the skillet and grill for 3-4 minutes or until golden brown and crispy.

Gently flip the sandwich using a spatula and grill the other side for another 3-4 minutes until crunchy and the cheese is melted.

Transfer the sandwich from the skillet, allow to cool for a minute before slicing in half. Serve warm, preferably with a bowl of tomato soup.