Scrub potatoes; pierce with fork. Arrange on paper towel in microwave. Microwave on HIGH for 3 to 5 minutes or until very tender when pierced with fork. Cool under cold running water. Halve potatoes and slice. Set aside.
In large bowl, combine lettuce, spinach and onion; toss gently. Arrange on 4 individual plates. Top with potato slices, tuna, celery and olives. Sprinkle each with pepper. Serve with salad dressing.