I shoulder of lamb on the bone
2 carrots, cut into 1cm dice
1 celery stalk, cut into 1cm dice
1 large onion, cut into 1cm dice
8 garlic cloves, finely chopped
A sprig of rosemary, eaves finely chopped
Leaves from 4 sprigs of thyme
2-3 tablespoons Worcestershire sauce, to taste
3 tablespoons tomato ketchup
500ml chicken stock a handful of frozen peas
salt and freshly ground black pepper
For the Duchesse potato topping
6 large baking potatoes (about 2kg)
200ml double cream
20g unsalted butter
6 egg yolks
Season the lamb shoulder well and put it on a wire rack in a roasting tray. Cover the tray loosely with foil, place in the oven at 120°C/Gas Mark Y2 and slow-roast for about 8 hours - at Fortnum's they normally put it in at the end of a shift so it is ready the next morning. Pick the meat out, removing all fat and skin - it is a lot easier to do this while it is still warm. Shred it into decent-sized pieces.
Heat a spoonful of lamb fat from the roasting tray in a large pan, add the carrots, celery and onion and cook gently until soft. Stir in the garlic and herbs and cook for a couple of minutes longer. Add the Worcestershire sauce, tomato ketchup and the lamb and mix well. Pour in the stock and simmer for 15-20 minutes, until it has reduced but the mixture is still moist. Check the seasoning.
To make the topping, wrap the potatoes in foil and bake at 200°C/Gas Mark 6 for about an hour, until tender. Leave until cool enough to handle, then cut them in half, scoop out the flesh and push it through a potato ricer or a sieve into a bowl. Bring the double cream and butter to the boil in a pan and beat them into the potato. Season well, then mix in the egg yolks. Put the mixture into a piping bag fitted with a star nozzle. Put the lamb mixture into a large pie dish, pressing it right down. Scatter the frozen peas over, then pipe the duchesse mix on top, covering the filling completely. Place in an oven heated to 180°C/Gas Mark 4 for about 20 minutes, until piping hot and golden brown.