In a large mixing bowl, gently combine lean ground beef, plain dry breadcrumbs, minced onion, salt, Worcestershire sauce, egg, and grated Parmesan cheese. Shape into 30 meatballs and arrange on a baking sheet.

Preheat your oven to 400°F. Bake the meatballs for 20–25 minutes until browned.

Transfer cooked meatballs to a crockpot and pour spaghetti sauce over them. Set to low heat and cook for 4–6 hours.

Prepare sub rolls by slicing them open. Spoon 2-3 meatballs with sauce into each roll and top with mozzarella cheese. Broil for 2-3 minutes if desired.

Serve warm, optionally with potato chips or a fresh green salad.