Rinse the 320g of Japanese short-grain rice in a bowl of cold water, swirling and draining several times, until the water runs mostly clear, then drain well. Cook it with about 350ml water in a rice cooker or covered saucepan according to your usual method, or use 850–900g of ready-cooked rice instead, and start it before the topping so it finishes hot. Keep the rice covered and warm, then fluff it with a paddle just before serving so the grains stay tender and slightly sticky. That tender, lightly sticky texture is what lets the rice support the topping instead of turning pasty or falling apart under it.

Crack the 6 large eggs into a bowl, adding the 2 extra egg yolks if you want a richer, silkier finish. Beat very lightly with chopsticks or a fork, just enough to break up the whites and yolks, and stop while streaks of white and yolk are still visible rather than blending to a uniform yellow. Divide the egg between two small bowls or jugs, so each batch has its own half, ready to pour in two parts. That restraint is what gives the finished topping soft ribbons and custardy patches instead of a flat, evenly mixed set.

In a bowl or jug, combine the 500ml dashi, 80ml soy sauce, 80ml mirin, 2 tbsp sake, and 1½–2 tbsp sugar, then stir until the sugar fully dissolves. Start with 1½ tbsp sugar and taste, adding the rest only if the broth seems sharp, because it should be savoury, slightly sweet, and light rather than harshly salty. Keep the jug close to the stove, since the broth is used in two equal halves of about 350ml for the two batches. That balance is what keeps the finished bowl moist but never soupy.

Cook in two batches, each making 2 bowls, for better control of the egg. For the first batch, pour half the broth into a medium skillet or shallow pan, add half the sliced onion, and bring to a gentle simmer over medium heat. Simmer for 2–3 minutes, until the onion begins to soften and turn translucent, keeping the bubbles lazy rather than rolling. That gentle start is what softens the onion and sweetens the broth before the chicken goes in.

Add half the chicken, about 300g, to the pan in a single layer, nestling the pieces among the onion. Simmer for 5–7 minutes, turning once or twice, until the chicken is just cooked through, reaching at least 74°C in the thickest piece, and the onions are fully tender but still holding their shape. Lower the heat if the sauce begins to tighten or the bubbles turn aggressive. That single layer in a gentle simmer is what cooks every piece evenly while leaving enough sauce for the egg to set in.

Once the chicken is cooked, pour about two-thirds of that batch’s beaten egg evenly over the pan in a slow circle, so it lands across the chicken and onion. Cover and cook for 30–60 seconds, until the egg begins to set softly at the edges. Drizzle the remaining egg over the top, cover again very briefly, and remove the pan from the heat as soon as the egg looks softly custardy and glossy, with some slightly runny areas. That second pour is what adds layered, golden ribbons, and stopping early lets residual heat finish the set without drying it.

Divide the hot rice among 4 bowls, about 210g of cooked rice each. Spoon or slide the first batch of chicken, onion, egg, and a little of the sauce over two of the bowls, leaving the rice partly visible around the edges, and garnish with sliced scallions or mitsuba. Repeat the onion, chicken, and egg stages with the remaining half of each for the last two bowls, then serve immediately. That small amount of sauce, with the egg still softly set, is what makes the topping sit gently over the rice instead of flooding the bowl.