Bring the water to the boil and add buckwheat, lentils and salt. Cover with a lid, turn heat down and simmer for 15 minutes. To prepare vegetables, heat 2 tbsp oil in a skillet and saute mushrooms for 5 minutes. Remove mushrooms from skillet and set aside. Add the 2 remaining tbsp oil to the skillet and saute vegetables in order listed above. Cover and simmer for 5 minutes. Preheat oven to 190 degrees C. Meanwhile, bring a pot of salted water to the boil. Add cabbage leaves and blanch for 1-2 minutes. Drain and rinse under cold water. Line tin with cabbage leaves, covering the bottom and sides. Leave enough overhang to fold over and cover the top. Combine buckwheat with the cooked vegetables and roasted nuts. Mix arrowroot and wholewheat flour with a little water and add miso, then stir into buckwheat mixture. Add the remaining ingredients and stir through. Spoon into cabbage-lined loaf tin and press down firmly, fold overhanging leaves over the mixture. Cover with foil and place the loaf tin into a baking dish with enough water to come halfway up the loaf tin. Bake for 45 mins or until firm to the touch. Cool slightly, remove foil and turn onto a serving plate. Cut into slices and serve with Lemon and Garlic Sauce.