Boil macaroni in water with the tablespoon salt until fully tender, adding peas for the last minute. Drain, rinse briefly under cool water and drain thoroughly.
Whisk mayonnaise, yogurt, lemon juice, Dijon, dressing salt and pepper in a large bowl.
Add macaroni, peas, celery and red onion and toss until coated. Flake drained tuna into bite-size pieces and fold it in gently.
Cover and refrigerate for at least 30 minutes before serving cold; the listed prep time includes this chilling.
Stir gently once more before portioning and refrigerate leftovers within 2 hours.