Leek and Mushroom Pie

Another lovely Nigel Slater recipe.
A mushroom pie in the shepherd's pie style, made with assorted mushrooms, Marsala, crème fraîche and tarragon.

Serves 4

small leeks 400g, untrimmed weight
olive oil 2 tbsp
butter 30g
mushrooms assorted 600g total
lemon zest and juice of 1
plain flour 2 lightly heaped tbsp
dry Marsala 3-4 tbsp
vegetable or chicken stock 300ml
tarragon a small bunch (3 tbsp)
crème fraîche 2 heaped tbsp

For the potato topping:
floury potatoes 1kg
butter 50g

Trim the leeks, removing any dark green leaves. Anything pale green or white is fine. Warm the olive oil and butter in a large casserole over a moderate heat. Thinly slice the leeks, then add them to the butter. Cover with a lid and cook, at a low to moderate pace, for 10-15 minutes until they are soft, but not coloured.

Sort the mushrooms, dividing them into firm (chestnut, portobello, button) and the more fragile, "wild" varieties that will take less time to cook.

Once the leeks start to soften, remove from the pan and set aside. Add the firm mushrooms, sliced or quartered where necessary, to the pan. Continue cooking until they are lightly coloured, adding more butter or oil as you think fit. Return the leeks to the pan. Add the lemon juice and zest, then the flour, and continue cooking for 2 or 3 minutes then pour in the Marsala, followed shortly by the stock. Leave to simmer.

Remove the tarragon leaves from their stalks, leave them whole then stir into the sauce with the crème fraîche. Check the seasoning, remove from the heat, then pour into a large baking dish, tuck in the raw fragile mushrooms (they will overcook if added during the initial cooking) and set aside.

Cook the potatoes, unpeeled, in boiling, salted water until tender, then mash roughly, skins and all, with the butter. Place in rough mounds over the mushroom sauce (don't be tempted to smooth it), then bake for 45 minutes at 200C/gas mark 6.