Prep 15 mins
Cook 1 hour
400g small leeks, trimmed and thinly sliced
2T olive oil
600g mushrooms assorted - sliced or quartered
1 lemon zest
2T plain flour
3-4T dry Marsala or sherry
300ml vegetable or chicken stock
2T crème fraîche
For the potato topping:
1kg floury potatoes
1. Warm olive oil and butter in a large casserole over a moderate heat and add leeks. Cover with a lid and cook, at a low to moderate pace, for 10-15 minutes until they are soft, but not coloured.
2. Sort the mushrooms, dividing them into firm (chestnut, portobello, and button) and the more fragile, "wild" varieties that will take less time to cook.
3. Once the leeks start to soften, remove from pan and set aside. Add firm mushrooms to pan. Continue cooking until they are lightly coloured, adding more butter or oil as you think fit. Return leeks to the pan. Add lemon zest, then the flour, and continue cooking for 2 or 3 minutes then pour in Marsala, followed shortly by the stock. Leave to simmer.
4. Remove tarragon leaves from their stalks, leave them whole then stir into the sauce with crème fraîche. Check seasoning, remove from 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.
5. Cook unpeeled potatoes in boiling, salted water until tender, then mash roughly, skins and all, with 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.
Optional: Sprinkle over some grated cheese for the last 5 or 10 minutes.