Using a sharp knife, first cut your cucumber in half lengthways. Discard one half, then slice the cucumber in half again into quarters. For this recipe, you will need an eighth, so you will need to slice down the length of the cucumber again by positioning your knife at a forty-five degree angle.
Prepare the tuna by first positioning your knife 1.5 cm in from the edge of the steak . Bring the heel of the knife through the tuna from the top, angling it slightly so that the tip points downwards. This will ensure a fluid, straight cut. Because tuna steaks are usually quite thick, you will need to cut your segment in half along the length. You should end up with two, roughly oblong strips of tuna with a width and depth of about 1.5 cm.
Lay a single, full Nori sheet onto your bamboo rolling mat with the guide lines running horizontally. Using your fingers, transfer 90g of cooked white sushi rice onto the Nori sheet.
Spread the sushi rice evenly across the surface of the Nori sheet, using your fingers to fluff it out to the edges. Leave approximately 1.5-2 cm of space at the bottom of the sheet, and around 6 cm at the top.
Next, take the cucumber slice and lay it onto the sheet above the rice, with the skin edge against the rice. Push it back gently so that it creates a small wall of rice against it.
Lay the two oblongs of tuna side by side on the bed of rice in the center. Use your fingers to very lightly press the tuna strips into the rice.
Holding the corners of the bamboo rolling mat, curl the bottom end of the Nori sheet up and over the contents. Roll it over until the end of the Nori sheet is in line with the cucumber, then press the sheet down so it adheres to the flesh. Compress the roll by pushing down on the mat gently with your fingers, compacting the contents.
Unfold the mat, then flip over the roll using your fingers until the remaining section of the Nori sheet envelopes the roll. Fold the mat over the roll once again, and compress to help the Nori sheet adhere along the top seam. This will achieve a teardrop shape with two straight, angled edges that meet a curve below.
Lay the sushi roll flat on its side. Dip your knife into a little water, allowing the droplets to run down the blade. Next, cut off both ends of the sushi roll. For a clean cut, start with the heel of your knife at the top of the roll, and bring it through in a single, fluid movement.
Divide the sushi roll into eight segments by first cutting it in half, then into quarters, and finally eighths. This method ensures your segments will all be of similar proportions.
To arrange your segments, simply turn them onto their sides and position them next to one another to form a circle, exposing the squares of tuna, and the cucumber center.