Mix the meat with the breadcrumbs, milk, garlic, salt and pepper, and knead the mixture together with you hands. Roll into heaping tablespoon-sized rolls.
Heat the olive oil in a large skillet over medium heat. Add the onions and cook for 1 minute. Add the meatballs between the onions and brown for 1 minute. (You might have to do this in two batches, depending on the size of your skillet.)
Pour in the jar of marinara and shake the skillet gently to mix. Put on a lid and allow to simmer for 20 minutes.
When ready to serve, cut each dinner roll in half. Place a handful of grated mozzarella on the top and bottom of each roll. Spoon a meatball with the sauce onto the bottom bun; top with the top bun. Serve immediately!