Open the canned tuna, drain most of the oil (reserve a little if you like extra flavor), and transfer to a medium mixing bowl. Use a fork to flake the tuna into small pieces.

Combine aromatics

Add the diced celery, diced red onion, chopped capers, and chopped parsley to the flaked tuna. Stir to combine evenly.

Make dressing and mix

In a small bowl whisk together the mayonnaise, lemon juice, and Dijon mustard until smooth. Pour the dressing over the tuna and aromatics. Fold gently until the mixture is evenly coated.

Season and chill

Season with kosher salt and freshly ground black pepper to taste. Mix once more, then cover and refrigerate for at least 15 minutes to let flavors meld (up to 2 hours).

Assembly & Garnish

Prepare salad bed

Divide the mixed salad greens or butter lettuce among 4 plates or a single serving platter. If using, drizzle a little extra-virgin olive oil over the greens and lightly season with a pinch of black pepper.

Plate tuna

Spoon equal portions of the chilled tuna mixture (about 1/2 cup each) onto the greens or onto toasted baguette slices/crackers for open-faced servings. If plating on greens, top each mound with a slice of tomato.

Garnish and serve

Garnish each portion with a lemon wedge. Serve immediately so the toasted bread stays crisp, or serve as a chilled salad. Offer additional crackers or baguette slices alongside.