In a pot of boiling, salted water, cook pasta until al dente, 7 to 8 minutes. Drain and transfer to large bowl to cool slightly.
While pasta cools, soak onion in cold water. Drain onion, then stir into pasta along with celery, celery leaves, jalapeño, and tuna.
Whisk together mayonnaise, buttermilk, lemon juice, relish, and mustard.
Toss with pasta mixture and season to taste. Refrigerate until cold, at least 30 minutes and up to 8 hours.
