Combine ground beef, ground pork, Italian seasoning, lemon zest, grated onion, garlic, egg, breadcrumbs, salt and pepper to a large mixing bowl. Stir to combine.
Take approx. 2 tablespoons of the meatball mixture at a time and form into a meatball. Set aside on a plate until all the meatballs are formed.
Bring a 12 inch cast iron skillet to medium-high heat. Add butter and let melt; swirl to coat the pan. Add meatballs and sear on all sides. Add marinara sauce and diced tomatoes. Cover and let simmer 15 minutes.
While the meatballs are simmering, combine the Herbed Garlic Butter in a small bowl.
Turn the oven to Broil.
Prepare the hoagie buns by ensuring they are sliced halfway open. You *can* remove some of the soft insides if you feel there is too much - this is optional. Spread some Herbed Garlic Butter in the inside of each hoagie. When the meatballs are done add 3 meatballs to each hoagie (Note: save any leftover marinara sauce for later - it will be our dipping sauce!). Place on a baking sheet and sprinkle generously with shredded mozzarella cheese.
Place under the broiler for 2-3 minutes - watch closely - once the cheese is melted and just starts to brown a bit, remove from oven. Sprinkle with fresh parsley and serve with the excess marinara sauce from the skillet so you can dunk your subs!!!
Notes
Herbed Butter: if you don't have fresh I would simply use a teaspoon or two of a dried italian seasoning here.
Meal Prep: make the meatballs ahead of time and store in refrigerator in an airtight container - when it is time to make dinner simply warm up the meatballs quick, assemble the subs and broil until cheese is melted!
Dairy-Free: to make this dairy-free use a dairy-free parmesan cheese in the meatballs, shredded dairy-free mozzarella cheese and vegan butter.
Gluten-Free: to make this gluten-free use gluten-free breadcrumbs in the meatballs and a gluten-free hoagie bun.
Servings: makes 5 meatballs subs, 2 servings per sub
Hoagie Buns: you can just use a hot dog bun if you want - in which case I would probably make the meatballs a bit smaller - as written, they are pretty big and meant for a bigger hoagie-style bun.