Mustard mash-topped ham and leek pie

Method

1. For the ham hock, place the hock into a large casserole and add the rest of the ingredients. Cover with cold water, bring to the boil and simmer for 2-3 hours until the hock is tender and the meat comes off the bone easily. Remove from the casserole and cool. Meanwhile, pour the stock through a sieve into a jug and reserve, discarding the contents of the sieve. Remove the skin and cut the meat into bite-size chunks.
2. Heat the olive oil in a large, heavy-based casserole. Add the onion, leeks, carrots and thyme leaves, and season well. Fry over a medium heat until softened slightly and just golden brown. Add the plain flour and stir in well.
3. Increase the heat, pour in the cider and 300ml of the reserved, hot ham stock, add the ham and bring to the boil. Reduce to a simmer and add half the crème fraîche. Season with salt (to taste) and black pepper and simmer for 10 minutes until the sauce has thickened nicely.
4. Meanwhile, cook the potatoes in a pan of boiling salted water for 10 minutes until tender, then drain. Add the rest of the crème fraîche, mustard, 50g of butter and the milk. Season well and mash until smooth.
5. Spoon the pie filling into a large, ovenproof dish. Spoon over the mash, using a fork to spread it evenly, then dot with the rest of the butter.
6. Cool completely and cover with buttered baking paper, cling film and foil. Freeze for up to 3 months.
7. When ready to eat, defrost the pie completely. Preheat the oven to 200°C/fan180°C/gas 6, remove the foil, cling film and paper, and bake in the oven for 25-30 minutes until golden and bubbling.