Drain the tuna thoroughly in a fine-mesh strainer, pressing gently with the back of a spoon to remove excess liquid and oil—this prevents a watery, separated salad. While the tuna drains, finely dice the celery into 1/4-inch pieces for consistent texture, mince the red onion, chop the parsley, and if using capers, roughly chop them. I like to keep my celery pieces uniform in size so every bite has the same pleasant crunch and no large woody pieces.

In a medium bowl, whisk together the mayonnaise and Dijon mustard until well combined—the mustard not only adds a bright tang but also helps emulsify the dressing for a creamier texture. Add the drained tuna and gently fold it in with a spatula, being careful not to break up the tuna too much and lose its desirable flaky texture. Add the prepared celery, red onion, parsley, and capers, then season with a pinch of salt and pepper to taste. I always start with less salt since the tuna and capers already contribute some, then adjust at the end.

Transfer the tuna celery salad to a bowl for serving, or use it as a sandwich filling between two slices of bread, or serve it over crisp lettuce for a lighter option. The salad can be eaten immediately while the celery is at its crunchiest, or refrigerated in an airtight container for up to 2 days.