Method
1. To make filling, heat butter or oil in large pan and gently fry onion, carrots, celery and garlic for 15 minutes until soft. Turn up heat, add mushrooms, cook for three to four minutes. Sprinkle in thyme and add lentils. Pour over wine and stock, then simmer gently, uncovered, for 40 to 45 minutes until lentils are soft. Add splash of water if needed.
2. Meanwhile, boil potatoes until tender. Drain. You can use a potato ricer or mash with butter and milk, then season with salt and pepper.
3. Remove lentils from heat, add tomato puree and Marmite, if using. Stir well, check seasoning and grind black pepper over.
4. Heat oven to 200c/180c fan/gas 6. To assemble, butter four-six small pie dishes or one large dish and spoon in lentil mixture, then top with mash. Bake for 30 to 40 minutes until golden. Serve with peas, if you like.