I follow the recipe for the Skinnytaste meatballs.
Wet bread with water then mash up with your hands. Add to large bowl and combine with chopped beef, chopped spinach, egg, garlic, parsley, grated cheese, salt and pepper. Mix all ingredients well until thoroughly combined. Using a 1/4 cup measuring cup, measure meat, then divide in two so that each meatball is 1/8th of a cup. Roll into little meatballs.
Meanwhile, in a large nonstick skillet, add 1 tbsp olive oil on low heat. When oil is hot add as many meatballs that will fit, cook on low, turning often so that all side get browned. Cook until the centers are cooked through. When finished, place on a dish lined with paper towels to blot any excess oil. Drop meatballs into sauce and continue cooking the remaining meatballs, repeating the process. When all meatballs are in the sauce, simmer for an additional 15-20 minutes.
For the subs:
Slice the rolls lengthwise and place 3-4 meatballs on the buns and top with shredded mozzarella, if desired. Place under the broiler until cheese is melted and golden.