Place the chicken, water, salt and pepper in Dutch oven. Cover and bring to a boil over moderate heat. Skim as needed. Reduce the heat and simmer for 1 hour. Add celery root, leek and carrot to soup, simmer for 15 minutes  longer. remove chicken from broth and cool slightly. Remove skin and bones, then dice chicken meat. Return diced chicken to soup. Add parsley, peas, and mace, simmer for 8 - 10 minutes. Beat the egg yolks and cream together well in small bowl. Add 2 tablespoons of hot soup to cream mixture, stirring constantly. Slowly add cream mixture to soup. Stirring constantly cook over very low heat for 3 minutes. Ladle soup  into serving bowls. Sprinkle each bowl with shredded lettuce. Serves 6.