Sprinkle gelatin on water in saucepan. Place over medium heat, stirring constantly, until gelatin is dissolved, about 3 minutes. Remove from heat.
Add soup, salt, mustard, ¼ c. lemon juice, onion and salad dressing. Beat with rotary beater until smooth. Chill, stirring occasionally, until mixture mounds when dropped from spoon. Add tuna and celery.