Heat the oil in a deep heavy-based pan until a breadcrumb sizzles and turns brown when dropped in it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Mix the flour and water together in a bowl until the batter has the consistency of single cream.
Add the ice cubes to chill the batter.
Dip the prawns and spring onions into the batter, then add to the hot oil to deep-fry for 4-5 minutes, or until golden-brown and crisp. Remove with a slotted spoon and drain on kitchen paper.
For the salad, mix the oil and balsamic vinegar in a bowl. Add the spinach and mix to coat the leaves in dressing.
To serve, place the spinach into the centre of a plate. Arrange the spring onion and prawn tempura on top of the spinach.