Remove string cheese from wrappers and cut each string into 8 equal pieces. Place in a large bowl or 1 freezer bag and freeze until it is time to form meatballs.
Make the meatballs
Using your hands, thoroughly combine beef, breadcrumbs, eggs, onion, garlic, salt, and pepper in a large bowl.
Shape into 1 inch meatballs, around each piece of frozen mozzarella.
Seal meatballs well to keep the cheese from oozing out.
Cook the meatballs
Heat olive oil in a large skillet over medium-high heat.
Cook the meatballs, turning often until they are browned on all sides
Drain any fat, then add the marinara sauce to the skillet and stir to combine with the meatballs.
Simmer, covered with a lid for 8-10 minutes until meatballs are cooked through.
Serve with pasta.
To oven bakePreheat oven to 450 degrees F. Brush a rack with olive oil and place meatballs on rack on rimmed baking sheets and bake at until fully cooked (about 15 to 20 minutes)To freezeLay meatballs on a baking sheet and freeze until solid, then add them to a ziplock bag ready for whenever you need 6, 12 or 20 of them! To cookRemove meatballs from freezer and reheat in the microwave, or in your favorite marinara sauce.