Divide the chicken, shiitake and mitsuba among four ceramic cups and pour the egg mixture. Cover each cup with aluminum foil and place on the steaming plate, then cover the lid. Turn the control lever to HIGH, and once it reaches a boil (or when steam begins to rise from the gap between the pan and the lid), reduce heat to SIMMER and steam for 5 minutes. Check the doneness, and if the custard is still runny, steam another 5 minutes. Repeat until the custard is firm. Insert a toothpick into the custard. If the toothpick comes out clean, it's done. Turn the control lever to off and remove the cups.