River Cottage

Preheat the oven to 220�C/Gas 7. Put the lamb in a large roasting tin. Trickle with a little olive oil and season well with salt and pepper. Roast for 25–30 minutes until golden. Meanwhile, peel the potatoes and cut into slices no thicker than a �1 coin.

Remove the lamb from the roasting tin and set aside on a plate. Turn the oven down to 150�C/Gas 2. Splash about half a glass of hot water into the hot roasting tin and use a spatula to loosen any bits stuck to the bottom. Spread a layer of potato slices over the base of the roasting tin, on the meat juices. Don’t bother to make it too neat.

Scatter over a little of the onion, garlic and rosemary, if using, then add a layer of mushrooms. Season with salt and pepper. Repeat this process to use up all the ingredients – aim for two or three layers, finishing with potatoes on the top.

Place the lamb on top of the potatoes. Cover with foil, sealing the edges well. Return to the oven and cook undisturbed for 3 hours.

Transfer the lamb to a warm plate to rest. The veg underneath should be completely tender – cooked in the juices and fat from the roasting lamb and transformed to a rich, meaty-flavoured potato and mushroom gratin. To top it off with a lovely crisp golden crust, preheat the grill to high. Put the tin of veg under the grill for 5–6 minutes until golden brown and crisp on top.

Serve the lamb thickly sliced with the potatoes and some simply steamed greens, such as leeks and kale. Or a sharply dressed leafy salad will go well with, or after, the meat and potatoes.

Swaps Try a small rolled shoulder of pork, instead of the lamb, and thinly sliced fennel instead of the mushrooms. Rub a few crushed fennel seeds into the pork skin, before the initial hot roasting, if you like.

• Taken from Hugh's Three Good Things available here>