In a heavy-based casserole pot over a medium heat, colour the mutton in batches until golden brown. Remove with a slotted spoon and set aside. Add the sliced onion to the fat which the mutton has created, and sweat on a medium heat until soft and sweet but not coloured. Add the potatoes, carrots, turnips and garlic. Cook for a further 5 minutes.
Preheat the oven to 140°C / fan 120°C / gas 1. Put the mutton back in with the vegetables along with the herbs and the red wine. Reduce the wine by half and then add the stock. Bring to a simmer, season and then cover the pot. Cook in the oven for approximately 2 hours or until the mutton is tender.
To make the pastry, tip the self-raising flour and suet into a large bowl and season. Steadily add the water, mixing with a wooden spoon until it starts to come together. Now pull it together with your hands to form a dough, trying not to work the pastry too much in the process. Wrap the dough in clingfilm and place in the fridge for at least half an hour.
To make the pie, take the mutton mixture out of the oven and transfer to a lipped pie dish so that the filling comes right to the top. The mixture should be quite wet as the mutton braising liquid is an essential part of the dish. Turn the oven up to 180°C / fan 160°C / gas 4.
Now roll the pastry out so that it is about 2cm thick and wide enough to cover your pie with an inch to spare. Brush the lip of the pie dish with beaten egg and stick the pastry lid on top, making sure it's completely sealed all around. Brush the top with more of the beaten egg and bake in the oven for approximately 1 hour until golden brown.
James Ferguson is head chef at Beagle restaurant, London