Preheat oven to 400°. Make meatballs: In a large bowl, combine ground beef, garlic, egg, bread crumbs, and parsley. Season generously with salt and pepper, then stir together until just combined. Form into 30 evenly sized balls.
In a large skillet over medium-high heat, heat oil. Cook meatballs, in two batches, until no pink remains, 5 to 6 minutes, then drain on a paper towel-lined plate.
Grease a 9″-x-13″ baking dish with cooking spray. Place baguette slices in an even layer in the bottom of dish. In a medium bowl, stir together melted butter, garlic, and Italian seasoning and season with salt and pepper. Brush all over baguette. Top with 1 cup mozzarella, an even layer of marinara, meatballs, and remaining mozzarella.
Bake until cheese is melty, 12 to 15 minutes.
Garnish with parsley before serving.