Bring the 275ml/9½fl oz of water to the boil in a saucepan. Put in the frozen spinach, cover and cook until it is just done. Drain in a colander and rinse under cold running water. Press out most of the liquid in the leaves (you do not have to be too thorough) and then chop them coarsely.
Cut the onions in half lengthways, then across into very fine slices.