Use a biscuit cutter to shape neat rounds of boulangère potatoes to serve with a mushroom-stuffed leg of lamb.
Equipment and preparation: you will need kitchen string and a round biscuit cutter.
For the stuffed leg of lamb, preheat the oven to 200C/400F/Gas 6.
To make the stuffing, heat a frying pan until medium-hot, add the butter and onions and fry until softened and very lightly browned, then add the tarragon and pour it all into a bowl and set aside.
Return the frying pan to the heat, and when hot, add the clarified butter and the lamb mince and fry until light brown. Season with salt and black pepper then drain, discarding any excess fat.
Tip the mince into the bowl with the onions and stir in the breadcrumbs.
Return the frying pan to the heat once more, add the mushrooms and fry in the residual oil until just tender. Season with salt and pepper and add to the bowl.
Mix everything in the bowl together, check the seasoning and leave to cool slightly.
Open up the leg of lamb, season the inside with salt and pepper. Place the stuffing in the centre, then fold the lamb back over pressing down lightly to just seal together.
Tie at regular intervals with kitchen string.
Heat a large frying pan until hot, brown the lamb on each side and then remove from the pan.
For the boulangère potatoes, place the potatoes and onions in a bowl, toss together and season with salt and freshly ground black pepper.
Layer the potatoes and onions into a buttered ovenproof dish and pour over the stock.
Place the lamb in the oven, directly on a rack near the top of the oven and place the dish of boulangère potatoes on the rack underneath to catch the lamb fat and juices as it cooks (alternatively place the lamb on a trivet over the potatoes). Cook for 15 minutes, then turn the oven down to 180C/350F/Gas 4 and cook for about 1-1¼ hours depending on how you like your lamb cooked.
When the lamb is cooked, remove from the oven and place onto a plate to rest for 20-30 minutes, then remove the string.
Keep the potatoes in a warm place covered with foil.
To serve, using a round cutter, stamp out circles of boulangère potatoes and place in the centre of each serving plate.
Carve the lamb and lay several slices just over the potatoes, then spoon any juices over the top.
