Preheat the oven to 180 C / Gas 4. Grease a 23x13cm (9x5 in) loaf tin, then line with baking parchment.
Start beating the egg whites to soft peaks. Add 4 tablespoons of icing sugar. Beat the egg whites to stiff peaks, but don't let them get too dry. Set aside.
In a separate bowl, beat the egg yolks and caster sugar until pale and thick, about 4 minutes. Whilst beating, add the lemon juice and then the olive oil in a slow, steady stream. Add the lemon zest.
Stir flour into the yolk mixture using a spatula or wooden spoon. Gently fold in the egg whites till no streaks of white remain. Tip into prepared tin.
Bake in the preheated oven for 40 to 50 minutes, or until a skewer inserted in the centre comes out clean. Cool in the tin for 5 minutes, then remove from the tin to cool completely. Dust with icing sugar, if desired.