In a large bowl, combine ground beef, shallot, garlic, parsley, bread crumbs, Parmesan and egg. Using a wooden spoon or clean hands, stir until well combined. Roll mixture into 1 1/4-to-1 1/2-inch meatballs, forming about 18-20 meatballs.
Heat olive oil in a large skillet over medium high heat. Add meatballs, in batches, and cook until all sides are browned, about 4-5 minutes. Transfer to a paper towel-lined plate.
In a small bowl, whisk together tomato sauce, basil, garlic powder, red pepper flakes and sugar, if using.
Reduce heat to low and add tomato sauce and meatballs to the skillet, simmering until the sauce has thickened, about 10-15 minutes.
To assemble the sandwich, place 3-4 meatballs and a spoonful of marinara into each baguette. Top with sliced mozzarella and place into oven and bake until the cheese has melted, about 5 minutes.