2 cups medium-grain white rice (such as Calrose)
2 1/2 cups cold water
1 (4-inch-square) piece dried kelp (konbu),* wiped lightly with dry cloth
1/4 cup rice vinegar
2 tablespoons sugar
2 teaspoons kosher salt
Spicy Tuna Mixture:
1/4 cup mayonnaise
2 tablespoons Dijon mustard
1 tablespoon chipotle pepper puree
1 tablespoon honey
2 tablespoons canola oil
Salt and freshly ground black pepper
4 (8 1/4 by 7 1/4-inch) sheets roasted nori
1 cup white sesame seeds
1/2 Kirby cucumber, peeled, seeded, and cut into 1/16-inch-thick matchsticks
1 1/4 pounds fresh tuna, finely diced
1/2 cup coarsely chopped cilantro leaves
3 scallions, thinly sliced
For the rice:
Place the rice in strainer. Rinse under cold water until water runs clear. Drain well.
Transfer rice to heavy medium saucepan. Add 1 1/4 cups fresh cold water and kelp to pan. Cover and let soak 30 minutes. Uncover, discard kelp and bring mixture to boil. Reduce heat to low. Cover and cook until water is absorbed and rice is just tender, about 15 minutes. Remove from heat. Let stand covered 15 minutes. Transfer rice to large glass bowl.
Combine vinegar, sugar and salt in small saucepan. Stir over low heat until sugar dissolves. Drizzle mixture over rice. Gently toss rice with vinegar mixture to coat. Cover rice with clean damp towel and cool completely at room temperature (do not refrigerate).
For the spicy tuna mixture:
Whisk together the mayonnaise, mustard, chipotle, honey and oil; season with salt and pepper.
Place sushi mat on a work surface with slats running crosswise. Arrange 1 sheet nori, shiny side down, on mat, lining up a long edge of sheet with edge of mat nearest you. Using damp fingers, gently press 1/4 of rice onto nori in 1 layer, leaving a 1 3/4-inch border on side farthest from you. Sprinkle with sesame seeds.
Flip the nori and rice over so that the nori is now the top layer. Make a thin line of cucumber along the nori 1 to 2 inches from the edge closest to you. Drizzle some of the spicy mayonnaise mixture over the cucumbers, then spoon on the tuna and sprinkle with cilantro and scallions. Roll tightly, using the sushi mat to guide you. Slice about 1/1/2 inches thick. Repeat with remaining ingredients to make a total of 4 rolls.