Fry the lamb in a nonstick pan, starting on a low heat, until it gives off some fat, then turn the heat up to brown. Add the onion, carrots and celery, and cook with a lid on for 5 minutes.
Sprinkle in the flour and stir well until it absorbs any fat and coats the lamb, then add the stock, soy sauce, tomato purée and parsley. Season with pepper and bring to a simmer. Cook for 20 minutes with the lid on, then take the lid off and keep cooking until you are left with a thick mixture and any excess sauce has been bubbled off.
Meanwhile, boil or steam the potato until tender and mash thoroughly with most of the butter and some seasoning.
Heat the oven to gas 6, 200ºC, fan 180ºC. Pour the lamb mixture into an ovenproof dish, top with the mash, dot with the remaining butter and cook for 20-30 minutes, until the top is crisp and the lamb is bubbling.
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Freezing and defrosting guidelines
Cook as instructed and allow to cool completely. Then transfer to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. To reheat and serve, place on medium heat, stirring occasionally until the dish is heated through.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.