- Preheat the oven to 200°C/ gas 6. For the topping, peel and roughly chop the sweet potatoes and add to a pan of boiling water, cooking for about 15–20 minutes, until soft.
- Drain, then season with salt and pepper, and beat in the mustard and milk with a wooden spoon until the mash is smooth and creamy.
- To make the filling, add the oil to a pan, throw in the onion and cook for 5 minutes, stirring regularly.
- Add the carrot and mushrooms and cook for a further 5 minutes until the onions are golden brown.
- Now add the thyme, tomatoes, puy lentils, pepper, vegetable stock and soy sauce. Stir, then bring to a gentle boil, turn down the heat, cover and simmer gently for 5 minutes.
- Put the lentil mixture into an ovenproof dish and top with the potato.
- Bake for 15-20 minutes until golden brown. Serve with green vegetables.
- It's easy to make this dish gluten free by using gluten-free soy sauce, or tamari, a gluten-free vegetable stock cube and a suitable mustard.
- To get a crispy topping, use a fork to create lots of little peaks in the potato.
- Freezing instructions: Suitable for freezing once cooked. Defrost in the fridge or microwave and reheat thoroughly until piping hot throughout.