Follow directions for Boiled Sweetbreads, adding a small onion, carrot and celery to water for flavor. When tender and cool, trim and cut into ½ inch dice. Break walnuts into large pieces. Drain mushrooms and cut each mushroom in half Double the recipe for Boiled Mayonnaise, No. 3, and thin with plain or whipped cream. Have all the ingredients ice cold; when ready to serve, mix the first five, with the Mayonnaise. Serve on lettuce leaves and garnish with the yolks, riced lightly over the top.