Combine first 6 ingredients in a small bowl. Stir well; set aside.
Sprinkle pepper over salmon. Place salmon, potatoes, and onion in a vegetable steamer coated with cooking spray; sprinkle with dill. Steam, covered, 15 minutes or until potatoes are tender and fish flakes easily when tested with a fork. Break salmon into chunks.
Combine salmon, potatoes, onion, and celery in a large bowl. Add mayonnaise mixture; toss gently to coat.