Preheat the oven to 400°. In a small bowl, stir together the butter, garlic, Parmesan and parsley. Add the salt and generously season with pepper. 

Cut deep slits into the baguette, 1-inch apart without cutting all the way through, and place the baguette on a piece of foil. Liberally spread the seasoned butter inside the entirety of each slit. 

Wrap the baguette in the foil, place on a baking sheet and bake for 15 minutes. Remove the baguette from the oven and unwrap the top. If you’re adding mozzarella, use an offset spatula to gently open each slit and tuck a piece of mozzarella in. Return bread to the oven and bake until crisp and golden on top, another 5 minutes. Slice with a serrated knife (or tear with your hands) and serve at once.