For this recipe it has to be a British leg of lamb. I would recommend buying from a farmers’ market or a good local butcher. Ask about the breed, whether its organic or even the name of the farm. It’s good to get to know what’s available and learn about the different tastes. At the restaurant we use Grierson Organic Perthshire lamb and Richard Briggs Native Shetland Lamb - both different but both delicious. Don't overcook the leg, simply rub with oil and salt and lay on a trivet of roots with garlic and rosemary before roasting. And make sure you allow it to rest properly. Bring the whole joint to the table to carve, you'll get lots of oohs and aahs!
1 leg of excellent lamb, bone in
1 large onion, peeled and thinly sliced
1 bulb of garlic, halved
A sprig of rosemary
4 or 5 medium-sized potatoes, washed, skins on
2 leeks with outer layer removed, cut into 2-inch pieces
500ml good chicken or lamb stock
A few knobs of butter
25ml cold-pressed rapeseed oil
Good salt and pepper
Heat your oven to 200°C.
Firstly, rub the leg of lamb with half the rapeseed oil and sprinkle with a generous amount of salt and pepper.
Place about 1/3 of the onion on a roasting tray with the half bulb of garlic and the rosemary. Place the lamb on top and add a small glass of water (c100ml) to the tray.
Place the tray in the oven and roast half an hour, then turn the heat down to 150°C and cook for a further 45 minutes to an hour. After about 15 minutes add the leeks, season again and baste with the juices. Cook until the leeks are just soft (about 45 minutes), basting as you cook. Once it’s ready, remove from the oven and let it rest for at least 10 minutes.
Whilst the leg is roasting, fry the rest of the onion in the remaining oil for about 20 minutes on a low heat until softened and starting to brown. Season and keep warm.
Next, slice the potatoes. You want them to be very thin, so use a mandolin or slicer if you have one. Then start to assemble the boulangère. In a ovenproof dish, layer up the slices of potato and onion, pouring over the stock and seasoning each layer as you go. Add a few dots of butter on top and place in the oven for an hour to an hour and 15 minutes. Long enough for the potatoes to soften and soak up all the stock.
To serve, place the dish of boulangère potatoes on the table and carve the meat at the table, placing some leeks on each plate as you carve, and top with the cooking juices.
A wee tip: if you like garlic, squeeze the cooked, roasted garlic bulb onto the lamb as it is resting, before you carve.