Tip the flour into a large bowl and season with salt and pepper. Add the pieces of chopped lamb and toss in the flour until evenly coated.
Preheat the oven to gas 3, 170˚C, fan 150˚C. Heat 1 tablespoon of the oil in a large nonstick frying pan, add the meat and any excess flour and fry over a medium to high heat until it is lightly browned. You don’t want to overcrowd the pan, so fry the lamb in two batches if necessary. Remove the meat from the pan and set aside.
Pour the rest of the oil into the same pan and drop a small knob from the measured butter in with it. Tip in the onions, carrots and thyme then fry for about 5 minutes or until the onions are just starting to soften and the edges are turning brown.
Lay a single layer of the potato slices in the bottom of a 20cm diameter x 9cm deep (8in x 3½in) casserole dish. Arrange half the lamb over the potatoes. Tip half the onions, carrots and thyme over the lamb and spread out evenly. Tuck in half the mushrooms. Season, then repeat with the rest of the lamb and vegetables.
Arrange the remaining slices of potatoes over the top in overlapping layers (keep the neatest slices to go on the very top). Dot with small pieces of butter and season with salt and pepper as you layer. Press down gently if necessary to fit the potato slices in the dish (they will sink during cooking). Finish by dotting the last of the butter on top then grind over some pepper.
Pour the stock in at the side of the dish. Cover loosely with a sheet of lightly buttered foil, scrunching it against the sides of the dish. Put on a baking tray and bake for 1½ hours. Remove the foil and bake for a further 1 hour or until the meat is tender and the potatoes are golden. Let it sit for about 5 minutes before serving. Scatter with a few thyme leaves to finish.
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.