In a large mixing bowl, add the breadcrumbs and milk, stirring until the breadcrumbs have soaked in the milk. Then, add the cheese, egg, Italian seasoning, garlic powder, salt, and pepper. Mix until well combined.
Add the ground beef. Use your hands to mix well until all the ingredients are combined.
Line a baking tray with parchment paper. Then with wet hands, form about 20 meatballs — they should each be about 1 ½-2 inches in diameter.
Place the meatballs on the baking tray. Brush with oil and bake them for roughly 20 minutes, until brown and cooked through.
In a large skillet, add the remaining oil and heat over medium heat. Add the minced garlic and cook for 30-40 seconds before adding the canned tomatoes.
Give the tomatoes a good stir, then add the oregano, salt, pepper, and sugar. Mix well and turn down the heat, letting the sauce simmer for around 20 minutes, until the sauce thickens slightly.
When your sandwiches are ready to be assembled, split the sub rolls in half and place them on a baking tray or in an oven-proof dish.
Fill the rolls with meatballs, then spoon over the tomato sauce. Generously sprinkle the mozzarella over each sandwich.
Put the rolls under a broiler for around 5 minutes, until the cheese has melted. Sprinkle with fresh parsley, if desired.