Place all the sauce ingredients in a saucepan and bring to the boil. Cook on a low heat for 15 minutes, then strain. You can keep this sauce in the refrigerator for up to 3 weeks.
For the tempura batter, break the egg into a large bowl. Mix well with the water, then add the flour and lightly mix, leaving it slightly lumpy – this makes for a crispy batter. Set aside in the refrigerator.
Peel and devein the prawns, leaving the tail intact. Cut along the back and a little along the front so the prawns stay flat when they’re cooked. Cut the sweet potato into 6 mm (¼ in) slices, and the capsicum and eggplant into 4 pieces. Heat the oil in a saucepan to 170°C (325°F). Dip the vegetables in the batter and deep-fry until golden and cooked through, then drain on paper towels. Raise the temperature of the oil to 180°C (350°F). Holding the prawns by their tails, dip them in the batter one at a time and deep-fry until golden and just cooked, then drain on paper towel.
Serve the rice in large individual bowls, dressed with a tablespoon of the sauce. Place some shredded nori and the tempura on the rice, then drizzle over another