Place onion in a small bowl, and add red wine vinegar; set aside.
Whisk together lemon juice, mustard, honey, salt, pepper, and 3 tablespoons of the olive oil in a large bowl. Fold in tuna, olives, sun-dried tomatoes, celery, and basil. Drain onions, and fold into tuna mixture. If serving immediately, drizzle remaining 2 tablespoons oil over tuna salad, folding to coat. (Alternatively, cover tuna salad, and refrigerate until ready to serve. Drizzle with remaining 2 tablespoons olive oil, folding to coat just before serving.)