Preheat the oven to Gas Mark 4, 180℃, fan 160℃. Put the flour in a large bowl and season with salt and freshly ground black pepper. Toss the lamb in the seasoned flour so it's well coated. Heat 2 tablespoons of the oil with the butter in a large flameproof casserole dish or deep ovenproof sauté pan.
Tip in the lamb and fry over a high heat for 5 minutes, turning over occasionally, until no longer pink and starting to brown. Transfer to a plate with a slotted spoon.
Add the onion to the dish or pan with another tablespoon of oil if needed, and fry over a medium-high heat for 5 minutes, or until just starting to brown, stirring occasionally. Stir in the garlic and cook 1 minute, then the tomato purée and keep stirring for about 30 seconds. Pour in a little of the stock and scrape the bottom of the pan to deglaze it. Slowly add the rest of the stock, stirring all the time.
Return the lamb to the pan along with the carrots, potato and herb sprigs. Cover and bake in the oven for 13/4 hours. Taste and adjust seasoning as needed.
Just before the lamb has been cooking for 13/4 hours, make the dumplings. Put the flour in a medium bowl and season with salt and pepper.
Add the butter and rub together with your fingers to make rough crumbs. Stir in the rosemary, thyme and parsley, then pour in 2 tablespoons of cold water and mix using a round bladed knife to a softish dough. Cut the dough into 8 even-sized pieces and gently shape into small balls.
Drop these on the surface of the casserole and cook in the oven, uncovered, for 20 more minutes until the dumplings are risen and the lamb is meltingly tender. (For a thinner sauce, carefully stir in a little more stock or boiling water.) Serve scattered with parsley.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it 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. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.