Cooking lamb in hay gives it a delightfully earthy and smoky taste. Buy clean hay from a pet shop or garden centre.
For the boulangère potatoes, preheat the oven to 160C/325F/Gas 2.
Heat a heavy-based pan, add some oil, then add the sliced onion, leek, fennel, garlic, thyme, fennel seeds and fry for 3-4 minutes.
Meanwhile, bring the stock to the boil in a saucepan and grease a small ovenproof dish with a little of the butter.
Slice the potatoes thinly on a mandoline and start to layer up the dish with alternating layers of potato and the onion mixture, seasoning with salt and freshly ground black pepper in between.
For the final layer, arrange the potatoes in a neat layer and pour over the hot stock.
Add a couple of knobs of butter, cover with foil and bake for one hour. Remove the foil and cook for another 30 minutes, or until cooked and crisp.
For the lamb, turn the oven up to 200C/400F/Gas 6.
Heat a dash of olive oil in a cocotte (a cast iron casserole dish with a lid) placed over a stove top and brown the lamb all over. Remove the lamb from the cocotte.
Add the hay and a little more oil, just until the hay starts to smoke, then place the lamb on top and cover with the lid.
Bake in the oven for 12-15 minutes, or until the lamb is pink.
To serve, remove the lamb from the hay, rest for 5-10 minutes, then carve and serve with the boulangère potatoes.
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James Martin presents a compilation of favourite Saturday Kitchen moments.