Melt 25g butter in a large pan. Add the leeks and fry on a low heat for 3 minutes with the lid on. Add the mushrooms and garlic and turn the heat up a little. Fry for 2 more minutes with the lid off, until the mushrooms have a little colour.
Add the carrots, lentils, tomatoes, water and tamari and bring to the boil. Reduce the heat and simmer for 25 minutes.
Meanwhile place the potatoes in a pan of salted boiling water and bring back to the boil. Simmer for 15-20 minutes until the potatoes are soft.
Once the potatoes are simmering, preheat you oven to 220C / 200C / gas mark 7 / 425F.
When the potatoes are cooked, drain them and return them to the pan. Add the remaining 25g butter, plus the milk, cream, salt and pepper, and mash until soft and creamy.
Place the filling in a large oven proof dish and top with the creamy mash. Sprinkle over the cheese and place in your preheated oven for 15-20 minutes or until bubbling and brown on top. Serve just as it is or with extra veg, such as peas, sweetcorn or cabbage.
Nutrition information is approximate and meant as a guideline only.