Place the red onion into a small bowl. Pour three tbsp of apple cider vinegar over the onions and toss. Let sit for twenty minutes to allow the vinegar to take some of the oniony bite out of the red onion.
In the bottom of a large bowl, whisk together the remaining apple cider vinegar, EVOO, dijon mustard, salt and pepper.
Add the tuna, flake it with a fork and add the celery and drained beans to the bowl.
Drain the red onions, add to the bowl and toss to combine. Serve on a bed of baby romaine leaves. Can be refrigerated for up to a day.
If serving later, keep lettuce and tuna/cannellini bean mixture separate.