Drain the canned tuna thoroughly and flake it into your mixing bowl with a fork, breaking up any large chunks while keeping it light and flaky.
Finely chop the red onion. To mellow its sharpness, soak the chopped onion in a small bowl of cold water for about 5 minutes, then drain well.
Add the chopped onion and chopped celery into the bowl with the tuna. Gently fold the ingredients together to distribute evenly, keeping the tuna light and fluffy.
Squeeze the juice of half a lemon over the mixture, capturing the bright, zesty aroma as you do. Mix thoroughly to incorporate the citrus flavor.
Add the mayonnaise to the bowl and fold it in until the salad reaches a creamy consistency. Adjust the amount if you prefer it more or less rich.
Sprinkle the smoked paprika over the salad and gently fold it in, allowing the smoky aroma to infuse the mixture.
Taste the salad and season with salt and freshly ground pepper as needed. If using capers, chop them roughly and fold them into the mixture for an extra briny kick.
Give the salad a gentle final fold, ensuring all ingredients are well combined and seasoned to your liking. Let it rest for about 10 minutes to allow flavors to meld.
Serve the tuna salad chilled or at room temperature, garnished with extra lemon slices or fresh herbs if desired. Enjoy this vibrant, textured dish with bread, crackers, or as a filling for sandwiches.