I’ve used shiitake mushrooms but you could use your favourite mushroom – chanterelles, porcini, oyster would all work nicely.
Heat the butter and hazelnut oil in a large sauté pan over medium high heat. Add leeks and sauté until soft, about 3 minutes. Add mushrooms, rosemary and garlic and sauté for 2 minutes more. Add the potatoes and season with salt and pepper to taste and cook until potatoes are heated through. Add lemon juice and remove from heat. Serve immediately.