Preheat oven to 425 degrees F.
Place all of the meatball ingredients into a large bowl and mix thoroughly with your hands.
Line a large baking tray with parchment paper and shape your meatballs with your hands.
You should get approx. 20 golf ball sized meatballs. Place the meatballs in a single layer on your parchment-lined baking sheet pan and bake for 30 minutes.
While the meatballs cook, place the butter or oil into a large, deep sided pan.
Thinly slice your onions and cook them over medium-high heat until they are caramelized onions.
Add the beef stock and the garlic to the caramelized onion mixture, salt and pepper and simmer until the French onion sauce is reduced and thickened.
Nestle the golden brown juicy meatballs into the sliced onions sauce and top with the extra mozzarella cheese.
You can either broil the pan until the cheese melts or place a lid on it on the stovetop and cook on medium-low heat until the cheese melts.
Serve garnished with chopped parsley and any other fresh herbs you like.
Store leftover meatballs in an airtight container in the refrigerator for up to 5 days.