Using your hands, combine the minced beef with the breadcrumbs, parmesan cheese, garlic, egg, origanum, cumin and chilli and generously season with salt and black pepper.
Form the minced beef mixture into meatballs about the size of golf balls.To help firm up the meatballs, put them on a plate, cover and place in the refrigerator until needed.
Heat a large frying pan on a medium-high heat, pour a few tablespoons of olive oil in the pan and fry the meatballs for approximately 5 to 8 minutes or until they are golden brown all over.
To make the Napolitano sauce heat a splash of olive oil in a saucepan pan over a medium-high heat and fry the onions until they are soft and lightly golden. Add the garlic and grated carrot and cook for an additional minute.
Stir in the tomatoes, tomato paste and sugar, season with salt and freshly ground black pepper and bring to the boil.
Reduce the heat and place the meatballs into the sauce, gently simmer on a low heat for about 15 minutes or until the sauce has thickened and the meatballs are cooked through.
In the meantime, pre-heat the oven to 230°C/450°F or turn on the grill.
Using a sharp knife cut holes into the tops of each roll and hollow out some bread making sure the holes are big enough to fit one meatball into each roll.
Add the butter to a small bowl and melt in the microwave for a few seconds before whisking in the garlic. Brush the garlic butter into each bread hollow then spoon in some Napolitano sauce.
Nestle a meatball into each hole, sprinkle the mozzarella cheese over the meatballs and place in the oven under the grill to bake until the cheese is melted and golden, approximately 10 minutes. Serve whilst hot!