Put 2 cups short-grain sushi rice in a fine-mesh sieve and rinse under cold water, stirring with your hand, until the water runs clear (about 3–4 minutes). Drain and let the rice sit 20–30 minutes to absorb some moisture.

Cook rice

Combine the drained rice and 2.2 cups water in a rice cooker or a heavy pot. If using a pot, bring to a boil over medium-high heat, then reduce to low, cover and simmer 12–15 minutes until water is absorbed. Remove from heat and let rest covered 10 minutes.

Season rice

While rice cooks, warm 3 tbsp rice vinegar, 1.5 tbsp sugar and 1 tsp fine sea salt in a small saucepan over low heat just until sugar dissolves. Do not boil. Transfer cooked rice to a large shallow bowl or hangiri. Gently fold the vinegar mixture into rice with a rice paddle or wooden spoon using cutting and fanning motions to cool the rice to near room temperature (about 5–8 minutes). Cover with a damp towel to keep moist.

White tuna filling & topping

Prepare tuna

Keep tuna chilled until ready. For a chopped filling: dice 6 oz of the tuna into small uniform pieces. For sashimi-style topping: reserve 4 oz and slice thinly across the grain into 1/8" slices. Pat any excess moisture from the tuna with paper towel.

Make white tuna mix

In a bowl, combine diced tuna with 3 tbsp mayonnaise, 1 tsp soy sauce, 1/2 tsp sesame oil, 1 tsp lemon juice, 2 tbsp thinly sliced scallions and 1 tsp toasted sesame seeds. If you like heat, add 1/2 tsp Sriracha. Mix gently to avoid turning the fish into a paste. Taste and adjust seasoning; refrigerate briefly if needed until assembly.

Assembly & garnish

Prepare work station

Have your bamboo mat, nori, cucumber, avocado, wasabi, pickled ginger and soy sauce ready. Wet your hands with a mixture of water and a little rice vinegar to prevent sticking.

Maki roll (optional)

Place a nori sheet shiny-side down on the bamboo mat. With wet hands, press about 3/4 cup seasoned rice evenly over the nori leaving a 1-inch strip at the top. Lay a thin line (about 2–3 tbsp) of the white tuna mix across the center, add a few cucumber julienne and avocado slices. Use the mat to roll firmly, sealing the edge with a little water. Using a sharp wet knife, slice into 6–8 pieces, wiping the blade between cuts.

Nigiri-style (alternative)

Wet your hands and form 2-tbsp portions of rice into compact oblong mounds (about 8 pieces). Lightly smear a small amount of wasabi on each rice mound if desired. Drape thin slices of reserved sashimi-style tuna over the rice and press gently to adhere. For some pieces, top with a small spoonful (about 1 tsp) of the chopped white tuna mix for contrast.

Plate and garnish

Arrange rolls or nigiri on a plate. Garnish with extra sliced scallions, a sprinkle of toasted sesame seeds, pickled ginger and a small dollop of wasabi. Serve immediately with soy sauce for dipping.

Serving & storage

Serve fresh within 30–60 minutes for best texture. Keep any leftover prepared chopped tuna covered and refrigerated; consume within 24 hours. Keep unused sashimi-grade tuna chilled and use the same day if possible.