Soaking raw onions in cold water for 15 minutes helps mellow their flavor.
In a pot of boiling, salted water, cook pasta until al dente, 7 to 8 minutes. Drain, then transfer to large bowl to cool slightly.
While pasta cools, soak onion in cold water. Drain, then stir into pasta along with celery, celery leaves, jalapeno 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.