Stir the dashi, soy sauce, mirin, and sugar together. This recipe cooks in two batches in a 25 cm (10 inch) pan, which keeps the egg layer the right thickness.
Pour half the broth into the pan with half the onion. Simmer over medium heat for 3 minutes until the onion softens.
Add half the chicken in one layer and simmer 5 to 6 minutes, turning once, until just cooked through.
Beat 4 eggs lazily, about six strokes, so streaks of white and yolk remain. Pour three quarters of them over the simmering pan in a spiral, cover, and cook 1 minute until barely set.
Pour on the remaining egg, turn off the heat, cover, and wait 30 seconds. The top should be glossy and just barely holding together.
Slide the whole thing over two bowls of hot rice. Repeat with the second batch. Finish with scallions and shichimi.