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Heat the oil in a large pan, and fry the meatballs for about 8-10 mins, or until browned and cooked through. Set aside. Heat the oven to 180C/160C fan/gas 4.
Carefully split the whole batch of rolls through the middle, ensuring they remain attached at their sides. Lay the bottoms of the rolls in a roasting tin lined with a sheet of scrunched-up baking parchment or foil. Spread the tomato sauce over, then place one meatball on every roll. Sprinkle over the mozzarella, then cover with the tops of the rolls, pressing down gently.
Brush with the melted butter, then sprinkle over the oregano and parmesan. Cover with foil and bake for 20 mins, uncovering for the final 5 mins so the tops become golden.