Rinse the 300g of sushi rice under cold running water 4–6 times — more rinse cycles than other rice types because sushi rice has an exceptionally high surface starch content, and incomplete rinsing produces visibly cloudy water that turns into a gummy, over-sticky cooked rice rather than the slightly sticky but individually glossy grains that sushi rice requires. Rinse until the water running through the rice is nearly clear — a slight cloudiness at the final rinse is acceptable. Combine the rinsed rice with 480ml of cold water in a medium saucepan, bring to a full boil over high heat, reduce immediately to the lowest possible setting, cover tightly, and simmer for 15 minutes. Remove from heat and rest covered for 5 minutes. In a small bowl, combine the 45ml of rice vinegar, 15g of sugar, and 7g of salt — stir until the sugar and salt are fully dissolved before applying to the rice. Transfer the cooked rice to a large, wide non-metallic bowl — a wooden sushi tub if available, otherwise glass or ceramic. Pour the dissolved vinegar seasoning evenly over the rice and begin folding it through using a rice paddle or flat wooden spoon with a cutting-and-folding motion. Simultaneously fan the rice — a hand fan, a magazine, or the lid of the pot all work — while folding. The fanning is the technique element that separates correctly prepared sushi rice from simply vinegar-seasoned rice: the moving air rapidly evaporates the surface moisture from each grain as the vinegar is absorbed, producing the specific glossy, slightly tacky surface that is the visual characteristic of properly seasoned sushi rice and that prevents the grains from feeling wet. Spread the seasoned rice onto a baking sheet in an even layer and continue fanning intermittently while it cools to room temperature. Do not refrigerate — cold destroys sushi rice's texture irreversibly.

In a small bowl, whisk together the 80g of Japanese Kewpie mayonnaise, 15g of gochugaru, 10ml of sriracha, 5ml of lime juice, and 3g of minced garlic until completely smooth and uniform. The two-chili combination — gochugaru and sriracha — produces a more layered heat than either alone: gochugaru's fruity, slightly sweet, building Korean warmth provides the background heat that develops progressively through each bite; sriracha's sharper, more immediate, garlic-forward heat provides the front note. Together they produce a spicy mayo with significantly more aromatic complexity than plain sriracha mayo. Kewpie's egg-yolk-only, rice-vinegar-and-MSG composition makes it specifically richer, creamier, and more deeply flavoured than standard mayonnaise in this application — the resulting gochugaru mayo tastes specifically Japanese in character. Adjust: more gochugaru for deeper fruity warmth, more sriracha for sharper direct heat. Refrigerate until assembly.

Pat the 600g of sushi-grade tuna steaks completely dry on all surfaces with paper towels. Any residual moisture on the tuna's surface will immediately create steam on contact with the hot oil — preventing the sesame seeds from toasting and adhering rather than forming the firm, golden crust the recipe requires. Season both sides and all edges generously with the 4g of salt and 2g of black pepper. Pour the 40g of mixed sesame seeds — white and black combined — onto a flat plate and press each tuna steak firmly into the seeds on all surfaces, turning to coat every face and edge. Press with your palm rather than simply rolling the steak — pressing compacts the seeds into the seasoned surface and produces a more adherent, more complete crust that will not fall off during the sear. Heat the 20ml of neutral oil in a heavy-bottomed skillet — cast iron or stainless steel — over the absolute highest available heat until shimmering and just beginning to show wisps of heat. The oil must be genuinely at smoking-heat temperature when the tuna enters — if the temperature is insufficient, the sesame seeds will absorb oil and become greasy rather than toasting crisply. Place the tuna steaks in the pan and leave completely undisturbed for 45–60 seconds for rare — the correct doneness for sushi-grade tuna that produces a raw, deep-red-pink centre — or 90 seconds for medium-rare. Do not touch, press, or move the steaks during this time. The sesame seeds toast and set against the surface during the undisturbed contact; any movement before they have set will pull the crust off the tuna. Flip each steak carefully and sear the second side for the same time. Transfer immediately to a cutting board and rest for 2 minutes. Slice against the grain into 1cm thick pieces — the cross-section of each slice should show the raw, deep-red interior surrounded by the toasted sesame-and-seared exterior.

Divide the cooled sushi rice among four wide bowls — approximately 200g of cooked rice per bowl, spread across the base. Arrange the sliced tuna pieces in overlapping or fanned formation over each rice portion so the raw red interior and toasted sesame crust are both visible. Distribute the toppings in distinct sections around the tuna: the 200g of julienned Persian cucumber, 160g of shredded carrot, 120g of cooked edamame, and 240g of sliced avocado each in their own section — arranged rather than mixed, so each component retains its individual visual identity until eaten. Slice the avocado immediately before assembly — avocado's oxidation is rapid once cut. Drizzle approximately 25g of the gochugaru mayo over each bowl — extending it across the tuna and reaching the rice, so every component is within reach of the sauce in each spoonful. Add 5g of pickled ginger per bowl — its bright magenta colour and sharp vinegary freshness provide both visual contrast and the palate-cleansing function it serves in traditional sushi service. Scatter the sliced scallions, toasted nori strips, and toasted sesame seeds over each bowl. Serve immediately with additional soy sauce on the side.