1.Preheat oven to very hot, 220°C (200°C fan-forced).
2.Bring dashi, sauce, mirin and sugar to the boil in a large saucepan.
3.Add chicken and mushrooms; return to the boil. Reduce heat; simmer, uncovered, about 10 minutes, or until chicken is cooked through.
4.Cool chicken and mushrooms in cooking liquid 10 minutes. Remove chicken from pan; slice thinly. Reserve mushroom and dashi broth.
5.Meanwhile, cook noodles in a large saucepan of boiling water, uncovered, until just tender; drain. Rinse under cold water; drain.
6.Return broth to the boil. Divide noodles, chicken, bamboo shoots, spinach and hot broth with mushrooms among four 3-cup (750ml) ovenproof dishes.
7.Make small hollows among noodles in each dish; break 1 egg into each hollow. Cook, uncovered, in oven about 10 minutes, or until egg just sets.
8.Serve soup sprinkled with onion.
The most common dashi, a combination of both kelp and smoked, dried bonito(tuna) flakes, is available in concentrated liquid as well as in granule or powdered form. Make sure that the spinach leaves you use here are, in fact, true spinach, and not the coarser silver beet that is often sold as spinach.