Finely dice 1/2 cup celery and 1/4 cup yellow onion. Place in a small bowl and, if you want milder onion flavor, rinse briefly under cold water and drain well before adding to the salad.

Drain and flake tuna

Open the canned tuna and drain thoroughly. Transfer to a mixing bowl and use a fork to flake it into bite-sized pieces.

Make dressing

In a separate small bowl, combine 6 tablespoons Kewpie mayo, 1 tablespoon yogurt (optional, or omit for richer mayo-only), 1 teaspoon Dijon mustard, 1 tablespoon fresh lemon juice, 1 teaspoon salt, 1/2 teaspoon freshly ground black pepper and 1/4 teaspoon sugar if using. Whisk until smooth and homogenous.

Mix salad

Add the diced celery and onion to the flaked tuna. Pour the dressing over the tuna and gently fold with a spatula or fork until evenly coated. Taste and adjust seasoning — add more salt, lemon juice or pepper as needed. Chill 10–15 minutes if time allows to let flavors meld.

Buns & Assembly

Toast buns

Split the 4 brioche buns in half. Heat a skillet or griddle over medium heat. Spread about 1/2 tablespoon butter on each cut face (total about 2 tablespoons). Place buns cut-side down on the hot skillet and toast 1–2 minutes until golden brown and lightly crisp. Remove from heat.

Assemble sandwiches

Place one lettuce leaf on the bottom half of each toasted bun. Spoon roughly 3 ounces (about 3 heaping tablespoons) of the prepared tuna salad onto each bun bottom, spreading evenly. Top the tuna with 2 bread & butter pickle slices (or to taste). Add extra cracked black pepper if desired. Close with the top bun.

Serve

Cut sandwiches in half if desired and serve immediately while buns are still warm. Leftover tuna salad keeps covered in the refrigerator for 2–3 days.