Drain the liquid from the canned tuna and transfer the solids to a stainless steel mixing bowl.
Use a fork to flake the tuna into consistent, small-sized pieces.
In a separate small whisking bowl, combine the extra virgin olive oil, lemon juice, and Dijon mustard, whisking until a stable emulsion is formed.
Incorporate the diced red onion, diced celery, and minced parsley into the bowl with the tuna.
Pour the vinaigrette over the tuna mixture and toss with a spatula to ensure even distribution.
Season with salt and black pepper according to the specified measurements.
Serve immediately or store in an airtight container at or below 40 degrees Fahrenheit (4 degrees Celsius).