Trim 850g of leeks, discarding the tough dark green leaves, then slice them into finger-thick pieces. Melt 40g of butter in a deep pan or casserole over a moderate heat and add the leeks. Cover with a lid and cook for about 10-15 minutes without letting them colour. A regular stir as they soften will prevent them from browning.
Cut in half 150g of chestnut or brown cap mushrooms. Roughly chop 400g of tomatoes. When the leeks are soft, add the mushrooms and tomatoes to the pan. Continue cooking, still covered with the lid. Preheat the oven to 200C/gas mark 6.
Roughly chop the leaves from 20g of parsley. Pick the leaves from 15g of tarragon and roughly chop them. In a small pan, warm 250ml of double cream and add the herbs and a seasoning of salt and pepper, bring to the boil then immediately remove from the heat.
Stir 2 tbsp of plain flour into the leeks and mushrooms and continue cooking for a couple of minutes. Stir in the warm herb cream, then transfer to a baking dish.
Use a food processor to reduce 250g of plain flour and 125g of butter to fine crumbs. (If you prefer, rub the butter into the flour with your fingertips.) Stir in 100g of grated Cheshire cheese and 1 tbsp of chopped thyme leaves. Scatter the crumble over the filling and bake for 30-40 minutes until the filling is bubbling. Serves 4
If you are concerned about the leeks browning, place a piece of baking parchment over the top, then cover with a lid. The paper will encourage them to cook in their own steam and reduce the possibility of caramelising.
To cut down on pans, you can cook the leek and mushroom filling in an ovenproof dish that will work on the hob as well. An enamelled, cast-iron casserole will work very well.
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