Add the garlic, herbs, butter, and Parmesan to the bowl of a food processor and pulse until well incorporated. Alternatively, you could fold the ingredients together using a rubber spatula.
Spread the butter mixture down the inner length of both slices of the bread. Wrap the pieces in aluminum foil and put them onto a sheet tray. Bake until the bread crisps up and the cheese melts, about 10 minutes. At this point, remove the foil, sprinkle the slices with the mozzarella cheese and season with salt and pepper, to taste. Slide the bread back into oven until the cheese bubbles, about 5 minutes. Remove the bread from the oven to a cutting board and slice before serving.