Best Meatball Subs Recipe - How to Make a Meatball Sub

Ingredients

• 1/2 c. diced celery
• 1/2 c. diced carrots
• 1/2 c. diced onion
• 2 cloves garlic, minced
• 1 c. freshly chopped parsley
• 3/4 c. bread crumbs
• 1/2 c. freshly grated Parmesan, divided
• 1 large egg
• 2 tbsp. ketchup
• Kosher salt
• Freshly ground black pepper
• 1 lb. ground beef
• 1/2 lb. ground pork
• 4 hero rolls, sliced lengthwise
• 1 c. marinara, plus more for serving
• 1 c. shredded mozzarella