In a large mixing bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, garlic, parsley, salt, black pepper, Italian seasoning, and milk. Mix well until all ingredients are evenly incorporated. Form the mixture into golf ball-sized meatballs.
Cook the Meatballs
Heat olive oil in a skillet over medium heat. Add the meatballs and cook for about 8-10 minutes, turning occasionally, until browned on all sides. Remove from the skillet and set aside.
Prepare the Marinara Sauce
In the same skillet, heat olive oil over medium heat. Add minced garlic and cook until fragrant, about 30 seconds. Pour in the crushed tomatoes, red pepper flakes, oregano, salt, and black pepper. Stir and let simmer for about 10 minutes, then add the meatballs to the sauce and cook for another 10 minutes, ensuring they absorb the flavors.
Assemble the Sandwich
Preheat the oven to 375°F. Slice the Italian hoagie rolls open and butter the insides. Place them on a baking sheet and toast for 5 minutes until slightly crispy.
Place 3-4 meatballs in each roll, spoon some marinara sauce over the top, and sprinkle generously with mozzarella and Parmesan cheese.
Melt the Cheese
Place the assembled grinders sandwiches Italian-style back into the oven or under the broiler for 3-5 minutes until the cheese is melted and bubbly.
Serve and Enjoy
Remove from the oven, let cool slightly, and serve hot! Pair with a side salad, potato chips, or a bowl of soup for the ultimate meal.