Mix the ground beef, breadcrumbs, egg, minced garlic, salt, and black pepper in a bowl until evenly combined. Divide the mixture into 8 equal portions and shape each one into a finger about 3 inches long. Use a knife to press two shallow lines across the middle of each finger for knuckles. Press one almond sliver into the tip of each finger to form a nail. Place the fingers on a baking sheet and bake at 400 degrees for 15 minutes until cooked through. Warm the marinara sauce in a small pan over low heat. Split the slider rolls, add one meatball finger to each roll, and spoon marinara sauce over the top.