Here's a healthier version of a Japanese lunchtime favorite, made with brown rice. Brown rice is nutty and chewy, and the short-grain version retains the stickiness that people have generally come to expect from Japanese white rice.
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Combine the water and rice in a medium-sized saucepan and bring to a boil over high heat. Cover, reduce the heat to low, and cook until the rice is almost tender and the water has been absorbed, about 40 minutes. Set aside, covered, to steam for 10 minutes.
Meanwhile, whisk together the vinegar, soy sauce, and sugar in a small bowl until the sugar dissolves. Spread the cooked rice into a 9-by-13-inch baking dish. Drizzle the vinegar mixture over the rice. Cool for 15 minutes, tossing every few minutes and spreading the rice out again.
Press the sesame seeds into the tuna steak on a cutting board. You may need to put some on one side, press them gently in place, then turn the tuna steak over and press down gently while adding more to the other side, repeating until coated.
Heat a large skillet over medium heat. Swirl in the sesame oil, then slip the tuna steak into the pan. Cook for 3 minutes, then turn with a wide, flat spatula, keeping as many of the sesame seeds in place as possible. Continue cooking until rare, about 2 more minutes, or medium-rare, about 3 more minutes. Transfer the tuna steak to a cutting board and slice into thin strips.
Spoon the seasoned cooked rice into 4 individual serving bowls. Smear ½ teaspoon wasabi paste over the rice in each bowl. Top with the tuna slices and daikon rings.