Line a rimmed baking sheet with aluminum foil and spray with cooking spray.
In a large bowl, combine meatloaf mix, Panko crumbs, milk, Parmesan cheese, pepperoncini peppers, onion, 1 teaspoon Italian seasoning, 1 teaspoon minced garlic, 1/2 teaspoon crushed red pepper flakes, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Use your hands to mix it just until evenly mixed.
Shape into 14 to 15 meatballs and place on prepared baking sheet.
Broil meatballs for 8 to 10 minutes.
In a 6-quart slow cooker, combine tomato puree, tomato paste, remaining garlic and Italian seasoning, 1/2 teaspoon salt, 1/2 teaspoon pepper,and sugar.
Transfer meatballs to slow cooker. Cover and cook on LOW for 5 to 6 hours or on HIGH for 3 to 4 hours.Skim off any grease on top and serve with pasta.