Gather all ingredients and prepare the workspace.
Preheat a skillet over medium heat.
Assemble the sandwich: place a slice of sourdough in the skillet, layer with mozzarella, tomatoes, basil, and a drizzle of balsamic glaze, then top with another slice of bread.
Drizzle olive oil on the top slice of bread before flipping it.
Cook the sandwich for about 3-4 minutes per side, or until golden brown and the cheese has melted.
Once cooked, remove the sandwich from the skillet and let it rest for a minute before cutting.
Slice the sandwich and serve with additional balsamic glaze if desired.