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1
Combine dashi, mirin, and soy sauce in the ratio 4:1:1 in a small saucepan. Bring to a brief simmer and taste: the sauce should read sweet before saline, with no sharp or bitter edge. This is the tare. Set aside. Prepare individual portions; each bowl is assembled and cooked separately in a small pan.
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2
For each bowl, place a single-serve pan (an oval copper okonomi pan where available, or a 20 cm skillet) over medium heat. Add one quarter of the sliced onion and 80–100 ml of tare. Simmer until the onion is just translucent and has begun to soften, 2–3 minutes. For katsudon, lay the sliced tonkatsu over the onion at this stage; for oyakodon, add the raw chicken pieces, turn once, and cook until the exterior is opaque and no pink remains at the surface — internal cooking will continue.
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3
This is the step on which the dish lives or dies. Ensure the sauce is actively simmering at approximately 80–85°C. Pour roughly two-thirds of one beaten egg (2 eggs total per bowl; use two-thirds of the total volume here) in a slow, steady spiral over the protein and onion, covering the surface evenly. Do not stir. Watch for the egg to begin setting at the edges and around the protein — firm, pale curds forming at the perimeter — while the centre remains liquid, approximately 30–45 seconds. Reduce heat immediately. Pour the remaining one-third of the egg in a second spiral over the still-liquid centre. The sauce temperature should now be approximately 70°C. The second addition should remain visibly fluid and barely set — a soft, almost translucent layer over the firmer first addition.
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4
Remove the pan from the heat while the egg centre is demonstrably still fluid — it should tremble when the pan is gently moved. Slide the entire topping over a bowl of freshly cooked, steaming rice. Cover the bowl immediately with a lid or a plate and leave undisturbed for 30 seconds. Carryover heat from the rice and the enclosed steam will set the surface without overcooking the interior.
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5
Remove the lid and serve at once. The finished topping should be set at the surface — no pooling liquid — but yield to a spoon with a soft, barely-cohesive interior. For katsudon, inspect the crust of the tonkatsu: the underside should have partially softened and absorbed the tare, contributing body and umami, while some surface crunch is retained. A topping that is fully firm and dry throughout indicates overcooking.