In a large mixing bowl, add ground beef, breadcrumbs, Parmesan, egg, garlic, Italian seasoning, onion powder, salt, and black pepper.
Stir gently with a spoon or your hands until just combined. Don’t overmix or the meatballs will turn tough.
Roll mixture into small meatballs (about 1 1/2 inches each).
Pour 1 cup of marinara sauce into the bottom of your Crockpot.
Place the meatballs on top in a single layer.
Pour the remaining marinara sauce over the meatballs.
Cover with the lid.
Cook on LOW for 6–7 hours or HIGH for 3–4 hours until meatballs are fully cooked and tender.
Preheat your oven to 375°F.
Slice slider buns in half horizontally and place bottoms in a greased baking dish.
Spoon some marinara sauce on the buns.
Place one meatball on each bun, then top with more sauce.
Sprinkle mozzarella over the top of each meatball.
Add the top buns back on.
Mix melted butter with garlic powder and parsley.
Brush over the tops of the buns.
Cover with foil and bake 10 minutes, then remove foil and bake another 5 minutes until golden and cheesy.
Sprinkle with extra Parmesan before serving.