Preheat the oven broiler to low and line a baking sheet with parchment paper. Combine ground beef, breadcrumbs, egg, salt, pepper, garlic powder, and parsley in a bowl.
Form meatball mixture into balls.
Heat olive oil in a pan over medium heat. Sear meatballs for 2-3 minutes on both sides, or until golden brown.
Remove meatballs from the pan. Pour pasta sauce in and stir together.
Add meatballs back to the pan. Cover with sauce and let simmer on medium low for 10 minutes, until sauce is warmed and meatballs are cooked though.
While meatballs are cooking, slice baguette into thin slices.
Combine melted butter, garlic, and parsley in a bowl.
Place baguette slices onto a baking sheet. Brush with butter mixture on both sides. Broil for 2-3 minutes, or until golden. Flip slices over and bake for another 2-3 minutes.
Assemble your sliders by spooning some sauce onto your crostini. Then add meatball, mozzarella cheese, and another garlic bread crostini. We used a homemade vegan mozzarella, which was already a melted consistency. To make these with store bought mozzarella, place your crostinis back into the broiler for another minute or so before putting the top crostini on your slider. Enjoy!