Open the canned tuna and pour it into a fine-mesh sieve or colander. Press gently to remove excess liquid, then transfer tuna to a medium mixing bowl. Use a fork to flake the tuna into small pieces.

Prepare vegetables

Finely dice the red onion and small-dice the celery. Chop the parsley and, if using, roughly chop the capers. Add these to the bowl with the flaked tuna.

Mix dressing into tuna

Add the mayonnaise, Dijon mustard, and fresh lemon juice to the tuna and vegetables. Stir gently with a spatula or fork until evenly combined. Taste and season with salt, freshly ground black pepper, and crushed red pepper flakes if using. Adjust lemon, mustard, or mayo to reach desired creaminess and brightness.

Chill (optional)

For best flavor melding, cover and chill the tuna mayo in the refrigerator for 20–30 minutes. If serving immediately, let sit 5 minutes to allow flavors to meld.

To serve

For sandwiches: spread 2 to 1/2 to 3/4 cup of the tuna mayo on each slice of toasted bread (or assemble open-face with one slice), top with sliced avocado and a handful of salad greens, then press or top with the second slice of toast.

For salad bowls: place 1 cup mixed salad greens in each bowl, spoon 1/2 to 3/4 cup tuna mayo over greens, garnish with extra parsley and a lemon wedge on the side.

For a quick snack: serve tuna mayo with crackers or cucumber rounds. Squeeze a little lemon over the top right before serving if desired.