Place poaching ingredients in a skillet. Bring to a boil then reduce to simmer.
Carefully place fish pieces into the liquid until they're about ¾ or more covered in liquid. Poach about 6-8 minutes. Remove fish pieces and allow to cool completely.
When cool, break up the fish into small pieces (to desired consistency) in a large mixing bowl.
In a separate small bowl, combine mayonnaise, mustard, lemon, salt & pepper and whisk until combined.
Add mayonnaise mixture to tuna bowl, along with onion and celery. Mix well to combine. Add more salt, pepper or lemon, to taste.
Notes
If you have large pieces of fish, cut them into smaller pieces so they poach more evenly. Add more water or wine to the pan if the fish isn't sufficiently 'snuggled' in the liquid :)
Recipe by My Delicious Blog at https://www.mydeliciousblog.com/fresh-blackfin-tuna-salad/