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Heat oven to 190C/170C fan/gas 5. Cut the lamb into chunky 5-6cm pieces. Tip the flour onto a large plate, season with pepper and a little salt, and toss the lamb in the flour to coat evenly and completely. Set aside.
Heat 2 tsp of the oil in a large, deep frying or sauté pan. Put the onions, carrots, swede, thyme sprigs and bay leaves in the pan and season with pepper. Cook on a medium-high heat for 6-8 mins, stirring occasionally so it doesn’t stick, until the vegetables start to go brown around the edges – the bottom of the pan should be slightly brown and caramelised too. Pour in the stock; it should sizzle then bubble in the pan. Immediately remove the pan from the heat so the stock does not reduce in volume. Stir in the Worcestershire sauce.
Overlap a single layer of the potato slices in the bottom of a 2-litre casserole dish (about 20cm diameter x 10cm deep). Lay half the lamb over the potatoes, then spoon half the vegetable mix over the meat. Season with pepper. Sit the rest of the lamb on top, then tip the rest of the vegetables, herbs and all the liquid over the lamb. Press down with the back of a spoon, if necessary, to make room for the remaining potatoes. Start to layer up the potato slices in 3 tightly overlapping layers, seasoning each layer with pepper and brushing with oil as you go – save a bit of oil for the top layer. Cover the dish with a tight-fitting lid and bake for 10 mins.
Reduce the oven to 160C/140C fan/gas 3. Cook for a further 2 hrs until the lamb is really tender and the potatoes are cooked. Take the lid off and pop under the grill for 8-10 mins until the potatoes are golden and crisp around the edges. Remove and let the hotpot settle for 5-10 mins, then serve scattered with thyme leaves.