A winter dinner staple, serve with a side of steamed kale or spinach.
Heat the oven to 150 degrees. Place the onion, unpeeled garlic cloves and rosemary in a large roasting tin and arrange the lamb shanks on top. Season with salt and freshly ground black pepper and pour over the wine and 500ml water. Cover tightly with aluminium foil and then cook in the oven for 4 hours. Remove and leave to cool completely.
Lift out the meat and pour the cooking liquid through a sieve. Discard the solids and set aside the liquid. Remove the meat from the bones and roughly chop into 5mm pieces, removing any fat and sinew. You can prepare 24 hours in advance up to this stage.
Heat the oil in a large saucepan, add the leeks, celery, carrots and crushed garlic, and cook over a low heat for 20 minutes, stirring frequently to prevent the vegetables from burning. Stir in the flour and cook for 2 minutes. Gradually stir in the reserved cooking liquid and chopped tomatoes. Bring slowly to the boil, stirring until slightly thickened, then turn down the heat and simmer for 15 minutes. Add the lamb and simmer for 10 minutes, season with S&P.
Take off the heat and stir in the parsley. Put the meat mixture into a large oven proof dish and top with the mashed potatoes. Use a fork to create a pattern on top of the mashed potatoes.
Cook the pie for 25-30 minutes until golden brown in a 180 degree oven.