1 Marinate the chicken in the sake and soy sauce for about 10 minutes. Drain and set aside.
2 To prepare the Dashi Custard, break the eggs into a large bowl and stir them gently with chopsticks or a fork. Do not beat or allow bubbles to form in the eggs. Combine the Basic Dashi Stock or instant dashi mixture with the sake, soy sauce and salt in a saucepan and place over medium heat, stirring slowly to dissolve the salt. Heat until just before boiling, then quickly remove. Do not allow dashi mixture to boil.
3 Pour the dashi mixture while still hot in a slow, steady stream into the eggs, stirring gently to blend. Strain the resulting egg and dashi mixture through a fine sieve to remove any lumps.
4 Evenly divide the chicken, prawns, and vegetables among 4 small heatproof bowls. Pour the egg mixture over contents, leaving a ¾-in (2-cm) gap at the top of each bowl. Cover each bowl with foil or a lid.
5 Heat the water in a large saucepan with a steamer rack, and place the custard cups on the rack. Cover partially with a lid to allow steam to escape. Cook over medium high heat for 1 minute, then reduce heat to low and cook for another 15 minutes, until a knife comes out cleanly when inserted in the custard. Remove from the steamer, garnish with grated lemon peel, and serve immediately.