Two 6-ounce cans tuna (preferably packed in oil), drained
1/4cup mayonnaise
3 tablespoons soy sauce
2 tablespoons fresh lemon juice
3 scallions, whites and greens separated and chopped
1 celery stalk, chopped
1/2 teaspoon wasabi powder (or mustard powder), or to taste
Pepper
1 ripe avocado, peeled, pitted, and sliced
2 tablespoons sesame seeds
Instructions
Put the rice in a small saucepan with 3 cups water, the rice vinegar, sugar, and a large pinch of salt. Bring to a boil, then adjust the heat so the mixture bubbles steadily and enthusiastically without boiling over. Cook undisturbed for 5 minutes, then stir, cover, and cook, checking every few minutes, until craters appear on the surface of the rice and no water is visible when you tip the pan, another 5 to 10 minutes. Stir again, cover, and remove from the heat; let the rice sit.
While the rice is cooking, mix the tuna, mayonnaise, soy sauce, lemon juice, scallion whites, celery, and wasabi powder in a bowl and sprinkle with pepper. Taste and adjust the seasoning, adding salt if you like.
To serve, stir the rice with a flat wooden spoon or a rubber spatula, using a fast, scooping-and-folding motion and pressing down gently until the rice is sticky and cooled a little, just a minute or 2; taste and add salt if necessary. Divide the warm rice among 4 bowls, top with the tuna mixture and sliced avocado, and garnish with the sesame seeds and scallion greens.