Clean the leek and cut into 3 cm long pieces. Blanch in boiling water for 3 minutes, rinse in cold water and drain.
Heat the butter and olive oil and sauté the leek with the mushroom quarters and the coarsely chopped onion for 1 minute. Season with pepper and cardamom, deglaze with soy sauce and cook closed on a low heat for 5 minutes. Allow to cool slightly.
Mix in the grated apple, season with salt if necessary, and then dress with the lemon juice and olive oil.