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If you need to preheat your slow cooker then turn it on now. In a large hot frying pan, add half the oil and brown the lamb shanks all over, then transfer them to the slow cooker. This will take you about 10 mins and you may need to do it in batches. Add the remaining oil to the frying pan and fry the onion gently until it starts to turn translucent, then stir in the tomato purée and flour and cook for a minute.
Add the red wine and bring it to a boil, stirring the flour and purée into the wine until you have a smooth sauce around the onion pieces. Tip into the slow cooker. Pour the stock into the same pan and bring it to a boil, then tip into the slow cooker. Add the carrots garlic, bay leaves, thyme and parsley stalks to the slow cooker and put the lid on. Cook on low for eight hours or on high for four hours. Turn the shanks over at some point during the cooking.
After eight hours the lamb should be tender and starting to fall off the bone. If the sauce is too thin lift out the lamb and carrots and tip the sauce into a pan, boil it rapidly until it starts to thicken before adding the parsley.