Begin by attaching the mincer attachment to the front, slow outlet of your kitchen machine, fitted with the medium mincer plate. Place a large mixing bowl under the outlet to catch the mince.
Start on speed 1, slowly add the chunks of beef to the hopper on the top of the mincer and feed them down the inlet tube.
Once the beef has been minced, add the rest of the meatball ingredients to the bowl and mix well. Feed this mixture through the mincer again to evenly combine the ingredients.
With your hands shape walnut sized balls of mixture and place on a prepared board or tray.
Once the meatballs have been shaped, heat the vegetable oil in a frying pan over a medium heat and fry them in batches on all sides until browned. Remove to an ovenproof dish.
Pour olive oil into a frying pan over a medium heat (you can use the same pan as the meatballs). Add onion and garlic and sauté until softened. Pour in tomatoes, tomato purée and stock. Then add thyme, sugar and season to taste. Lower the heat and simmer until the sauce is reduced by around 1/4.
Pour the sauce over the meatballs in the dish, top with cheeses and bake in preheated oven for 15-20 minutes, until the cheese is melted and the sauce is bubbling.
Scatter with basil leaves and serve with crisp garlic bread and a green salad.