This dish is called Kakiagé, an inexpensive and informal style of Tempura. This is only one of many versions and it is a good way of using up small quantities of vegetables.
Roughly chop half the prawns. Cut the mushrooms into small cubes. Slice the white part from the spring onions and chop this roughly.
To make the tempura batter, in a medium mixing bowl, mix the cold water and eggs. Add the flour and baking powder, and very roughly fold in with a pair of chopsticks or a fork. Do not beat. The batter should still be quite lumpy.