2) Transfer the rice to a heavy medium saucepan. Add 300ml fresh cold water and the kelp to the pan. Cover and leave to soak for 30 minutes.
3) Uncover, discard the kelp and bring the mixture to the boil. Reduce the heat to low. Cover and cook until the water is absorbed and the rice is just tender, about 15 minutes.
4) Remove from the heat and allow to stand, covered, for 15 minutes. Transfer the sushi rice to large glass bowl.
5) Combine the vinegar, sugar and salt in a small saucepan. Stir over a low heat until the sugar dissolves. Drizzle the mixture over the sushi rice.
6) Gently toss the rice with vinegar mixture to coat. Cover the rice with a clean damp towel and cool completely at room temperature (do not refrigerate).
7) Whisk together the mayonnaise, mustard, chipotle, honey and oil; season with salt and pepper.
8) Place a sushi mat on the work surface with the slats running crosswise. Arrange 1 sheet of nori, shiny side down, on the mat, lining up a long edge of the sheet with the edge of the mat nearest you.
9) Using damp fingers, gently press 1/4 of the rice onto the nori in 1 layer, leaving a 4cm border on the side farthest from you. Sprinkle the rice with sesame seeds.
10) Flip the nori and rice over so that the nori is now the top layer. Make a thin line of cucumber along the nori 2cm to 4cm from the edge closest to you. Drizzle some of the spicy mayonnaise mixture over the cucumbers, then spoon on the tuna and sprinkle with coriander and spring onions.
11) Roll tightly, using the sushi mat to guide you. Slice into pieces about 5cm inches thick. Repeat with the remaining ingredients to make a total of 4 rolls.