To make good tempura at home make sure the batter and the vegetables/prawns are very cold. Also, because you want to drain the tempura well once cooked and not let it get soggy, I drain it on a cake rack in a baking dish above paper towels.
Chop the vegetables to size, prepare the prawns and put them all in the fridge.
Tempura Sauce: Put all ingredients in a saucepan and gently simmer for about 2 minutes. Strain and discard solids. Set aside.
Batter: In a medium size bowl, lightly beat the egg, add the water and mix it well. Add the flour and mix it using two chopsticks. Do not over mix it, leave a few lumps of flour.
Heat the oil in a wok or large pot. Dip each of the cold pieces into the cold batter and fry until the outside is golden and looking crispy.
The vegetables will first sink to the bottom of the wok (or large pot) and then will rapidly come to the top. Remove with a slotted spoon or Chinese strainer. Drain in a cooling rack set over a baking tray. Serve immediately with the sauce for dipping.
To flatten prawns make a shallow cut along the back and press down on the whole prawn to flatten it.