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To make the filling, heat the butter or oil in a large saucepan and gently fry the onion, carrots, celery and garlic for 15 mins until soft and golden. Turn up the heat, add the mushrooms, then cook for 3-4 mins. Sprinkle in the thyme and add the lentils. Pour over the wine and stock, then simmer gently, uncovered, for about 40-45 mins until the lentils are soft. If you find the mixture is drying out, you can add a splash more stock or water.
Meanwhile, put the potatoes in a large pan, pour over boiling water and boil for 15-20 mins until tender. Drain well. You can use a potato ricer at this stage to make them super creamy, or mash with the butter and milk, then season with salt and pepper.
Remove the lentils from the heat, add the tomato purée and yeast extract, if using. Stir well and check to see if you need seasoning, then add a good grinding of black pepper.
Heat oven to 200C/180C fan/gas 6. To assemble, butter four-six small pie dishes or one a large dish and spoon in the lentil mixture, then top with mash. It’s fun to pipe on the mash if you have a piping bag, but if not, you can spoon it on and fluff into peaks with a fork. Bake for 30-40 mins until golden. Serve with peas on the side, if you like.