Boil the potatoes and turnips in a large pot of salted water for 20-30 minutes, or until very tender.
Drain off the excess water and mash the turnips and potatoes together with half of the butter.
Cook the lentils in a pot with abundant water for 30 minutes, or until nice and tender.
Drain and set aside.
Heat the rest of the butter in a frying pan and sauté the onions, leeks and parsnips over low heat for about 10 minutes. Add the walnuts, thyme, oregano, the cooked lentils and the tomato.
Mix well and season to taste with the salt and pepper.
Place the vegetable mixture in a deep oven-proof dish and cover with a layer of mashed potato and turnip.
Sprinkle the grated cheddar cheese over the top and cook in the preheated oven for 30-35 minutes, or until the mash turns golden brown and a bit crispy.
Remove from the oven and let cool for 5 minutes before serving.
Recipe by Gluten free cooking at http://recipes.hellomagazine.com/glutenfree/20120723/winter-vegetable-pie/