Put chicken, 2 teaspoons salt, peppercorns, sliced onion, and celery leaves in large saucepan. Add water to cover. Cover pan; simmer until tender. Remove cooked chicken from bones; return bones and skin to broth. Cook until broth is reduced to 1 quart; strain, cool and remove fat. Cube chicken (you should have four cups); reserve.
Soak gelatin in 1/2 cup cold chicken broth. Heat 2-1/2 cups chicken broth; add gelatin mixture; stir until dissolved. Combine grated onion, lemon juice, mayonnaise or salad dressing, mustard, 2 teaspoons salt and pepper. Add to gelatin mixture; mix well.
Chill until mixture begins to jell. Fold in celery, parsley and cubed chicken. Turn into 7-cup mold. Chill several hours or until firm. Unmold on to a serving plate. Garnish as desired with greens.