Stir together the breadcrumbs and milk in a large bowl. Add egg, onion, Parmesan, vnegar, salt and pepper and stir to combine. Add beef and mix together with your hands until just mixed. Using a small cookie scoop, shape meatballs and set on a cookie sheet.
Heat oil in skillet and cook meatballs over medium heat. Turning them often until browned on all sides. Transfer to a plate.
Reduced heat to medium low and add sauce. Simmer for 2 to 3 minutes and return meatballs back to the skillet. Sprinkle with mozzarella. Cover and simmer for about 10 minutes. Sprinkle with fresh bazil and serve.
NOTE: You could also use ground turkey for your meat and leave out the spinach.