To prepare the dumplings boil and peel potatoes. Mash them as puree. They add 2 tbsp flour, eggs, crushed biscuits, salt and pepper. Arugula blanch in salted water, chop finely and add to potato mixture. Finally, add the crushed pistachios. Mix well. From the resulting mixture into small balls and roll them in flour. Put 2-3 liters of water with a little salt in a deep pan and let it boil. Drop balls in the boiling water. Dumplings are ready when you come to the surface. For the sauce, fry in heated pan with butter until golden 1 tbsp flour. Warm milk and soy sauce with temperature as fried flour and add them to the pan. Stir constantly until thickened. If the sauce in your lumps, you can strain. Once the sauce is slightly cooled, add lemon juice. The bottom of the pan pour the sauce on top and arrange the dumplings. Serve dumplings warm.