With sashimi-grade tuna and a few simple techniques, you can make this restaurant quality sushi at home in under an hour. Adapted from Sushi Cookbook for Beginners by Chika Ravitch.
Place the rice in a fine-mesh strainer set over a bowl. Rinse under cool water, stirring gently with your fingers. Once the bowl fills with cloudy water, lift the strainer and pour it out. Repeat this process, refreshing the water each time, until it runs clear.
Transfer the rinsed rice to a medium bowl and add the measured water. Let it soak at room temperature for 15 minutes.
While the rice soaks, stir together the rice vinegar, sugar, and salt in a small bowl until fully dissolved; set aside.
Pour the rice and its soaking water into a heavy-bottomed saucepan. Add the piece of kombu, cover, and bring to a boil over high heat. As soon as it boils, reduce the heat to low and cook for 10 minutes. Once the water is absorbed, take the pot off the heat, place a clean kitchen towel between the lid and the pan, and let it steam for 10 minutes.
Remove and discard the kombu. Transfer the rice to a wide bowl and drizzle with the vinegar mixture. Use a rice paddle to gently fold it through the hot rice, being careful not to press or stir too hard. Cover the bowl with a damp cloth and let the rice cool to room temperature before forming your nigiri.
To Make the Tuna Nigiri
Start with sashimi-grade tuna and keep it cold until you're ready to slice. If there’s any sinew or dark bloodline, trim it away. It can be tough or have an overly strong flavor. Using a very sharp knife, slice across the grain into pieces about ¼ inch thick. Angle the knife slightly and use a smooth, single motion for each cut. The slices should be 2 to 2½ inches long, just enough to cover the rice without hanging too far over the sides.
Wet your hands and scoop a heaping tablespoon of sushi rice into your palm. Gently shape it into an oval mound, something close to a slightly flattened football.
Spread a small amount of wasabi on the rice, if using, then lay a slice of tuna over the top. Lightly press the fish onto the rice so it holds together without compressing the shape. Place on a serving plate.
Repeat with the remaining rice, wasabi, and tuna. Add a few slices of pickled ginger to the corner of the plate or serve in a small dish alongside, and offer a shallow bowl of soy sauce for dipping. Serve right away.
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Notes
Use sashimi-grade tuna from a trusted source. Look for bright color and firm texture.
A sharp knife makes all the difference for clean, even slices.
If you can’t find kombu, skip it; your sushi rice will still turn out well.
Wet your hands with water and a splash of vinegar to keep rice from sticking.
Store leftover nigiri in the refrigerator for up to 24 hours. Best enjoyed fresh, but still good the next day.