Beer gives a definite character to batter. This beer batter will taste yeasty but, in fact, relies on the last-minute inclusion of whisked egg white for its lightness and crisp finish.
Mix all the ingredients together except the egg whites. Leave for 1 hour.
Whisk the egg whites until stiff, folding them into the batter (1 and 2, opposite). Folding in requires a combined rotating and cutting motion; it is not essential for every scrap of egg white to be completely incorporated.