Preheat the oven to 350 degrees F.
Toast the muffin until golden brown. Top the bottom half of the muffin with the mozzarella and tomatoes, and then sprinkle with the Parmesan.
Place the muffin on a baking sheet and bake until the mozzarella melts and the muffin is slightly golden, 7 to 8 minutes. Let it cool for a couple of minutes before serving. Place the basil leaf on top and close the sandwich. Serve warm.
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