In a large bowl, combine the beef, breadcrumbs, milk, garlic, onion powder, salt and black pepper. Shape the mixture into 24 meatballs.
In a large skillet over medium heat, add the oil and swirl around the pan to coat. Cook the meatballs in batches, being careful not to crowd the pan, until browned on all sides. Drain any excess grease.
Add all of the meatballs to the skillet and pour the marinara sauce over the meatballs. Cover and simmer for 20 minutes. Serve inside slider buns with a slice of provolone for a fun dinner idea, or on top of spaghetti noodles.