Open the canned tuna and drain thoroughly. Place tuna in a large bowl and use a fork to flake it into small chunks, breaking up any large pieces but leaving some texture.

Prep mix-ins

Finely dice the celery and mince the red onion. If using eggs, peel and chop the hard-boiled eggs into small pieces. Chop the parsley and slice the tomato if using.

Combine dressing

In a small bowl, whisk together the mayonnaise, Dijon mustard, lemon juice, and sweet pickle relish until smooth. This creates a balanced creamy-tangy dressing.

Mix salad

Add the diced celery, minced red onion, chopped eggs (if using), and chopped parsley to the flaked tuna. Pour the dressing over the tuna and gently fold until ingredients are evenly distributed. Fold just until combined to keep a light texture.

Season and adjust

Season the salad with salt, black pepper, and optional celery salt. Taste and adjust: add more lemon for brightness, more mayo for creaminess, or more mustard for tang. Chill for at least 15 minutes if serving cold to let flavors meld.

To Serve

If serving cold: place a lettuce leaf on four slices of bread or on split rolls, divide the tuna salad into four equal portions (about 1 cup each) and spread onto the lettuce. Top with tomato slices if desired and close sandwiches with remaining bread. Cut in half and serve chilled.

If serving warm: gently warm the prepared tuna salad in a skillet over low heat for 3–4 minutes, stirring frequently to avoid drying out (do not boil). Alternatively, build sandwiches and toast briefly in a skillet or panini press until bread is warmed and slightly crisp. Serve immediately.

Storage: Refrigerate leftovers in an airtight container for up to 3 days. If you added eggs, consume within 2 days for best quality.