Preheat the oven at 200°C. Cook the arracacha with a bit of salt using a pressure cooker for about 30’ (you can peel it before or just remove it after cooked, since it’ll be almost melting). Mash the roots with a fork or use a mixer, if you don’t like lumps. Add the butter and mix well. The texture changes completely! Check the seasoning.
Slice the leek finely. Cook it on a frying pan with a bit of olive oil and salt until it softens. Chop the mushrooms roughly. In case you’re using dry mushrooms, let them soak on warm water for 30’ before cooking. Put the mushrooms on the pan and cook a bit more. Add the soy sauce and the olive oil.
Pour the mushrooms on a tray. Spread the mash arracacha on top and take it to the oven for 25’ to 30’. Serve right away!