For the meatballs, place the onion, garlic, mince, seasoning and chopped rosemary into a large bowl and mix well. Using your hands, shape the mixture into golfball-sized balls.
Heat the olive oil in a frying pan and fry the meatballs for 6-8 minutes, stirring frequently, or until golden-brown on all sides.
For the sauce, add a little more oil to the pan with the meatballs and sauté the onion for 3-4 minutes. Add the garlic and cook for one minute. Add the mushrooms and sauté for 2-3 minutes.
Add the red wine, turn up the heat and simmer for 1-2 minutes. Add the tomato purée and beef stock and simmer for 8-10 minutes, or until the sauce is thickened and the meatballs are completely cooked through.
For the mash, place the cooked potatoes into a large bowl. Add the butter, cheese, milk and seasoning and mash together with a potato masher until smooth.
To serve, place the mash onto a serving plate and spoon over the meatballs and sauce.