To season the tuna, add a sprinkling of pepper to one side, followed by some crushed sesame seeds. You can either crush these yourself using a mortar and pestle, or using a spice grinder. For even coverage, pour the sesame seeds onto a flat plate and press the tuna steak into the sesame seeds.
Add a little coconut oil to a preheated frying pan. One or two tablespoons should be enough. Then, add a drizzle of sesame seed oil to your heated pan for flavoring.
Lay the tuna steak covered-side down in the oil, and sear at a consistent heat for approximately two minutes. The easiest way to check when it’s done is to look at the side of the tuna, which turns white as it cooks through. It should be around 1/3 to halfway cooked.
Remove the tuna fillet with a spatula and lay on a chopping board seared side up. This will ensure the heat does not become trapped between the board and the fish, cooking the rest of the tuna. The topping should be crispy and brown at this stage.
Cut the tuna lengthways into strips, approximately 1 centimeter (2/5 of an inch) in width, then set aside.
Place your bamboo rolling mat on the chopping board, followed by a sheet of nori, rough-side up. Then, take 150g (5 oz) cooked sushi rice, and using your fingers, spread across your nori sheet. Ideally, the rice should cover 70-80% of the sheet, with a 1-2 cm space left at the end for ease of rolling.
Cover the rice with a few fresh spinach leaves, leaving approximately 1 centimeter of space along the far edge, followed by a layer of Cole sprouts. Finally, position two of your seared tuna slices at the center of the bed of Cole sprouts.
To roll your sushi roll, curl your fingers just underneath the edge of the bamboo mat as you turn it over. Then, use your fingers to close it and compress the contents. Finally, roll the mat all the way over and compress again to close the roll. Leave the sushi roll resting on its seam for about two minutes. This will allow the moisture of the rice to sink into the seam of the sheet – thus helping it to seal.
Moisten the blade of your knife in a bowl of cold water so that the rice doesn’t stick when making your cuts. Angle your knife, allowing the droplets to run down and coat the blade.
Cut off both ends of the roll first and set aside – either for use as decoration, or as props for presentation.
Next, cut the roll in half, and then the halves into quarters. Divide these further into eighths – all of equal width. For maximum aesthetic impact, serve the segments flat on a plate with a central garnish, or arrange the segments like dominoes, using one of the discarded ends to prop up the first segment.