Whisk the egg whites in a large, clean bowl until stiff peaks form. Gradually whisk in the caster sugar, a spoonful at a time, until stiff and glossy. Whisk in the icing sugar, in 3 batches, until the meringue is stiff and resembles shaving foam. Fold in the lemon rind.
Spoon 15–16 small mounds of the meringue on to 2 baking sheets lined with baking paper, leaving space between each one. Sprinkle the almonds over the tops.
Bake in a preheated oven, 110°C (225°F), Gas Mark ¼, for 1¼ hours until crisp and dry and the meringues sound hollow when tapped on the bottom. Leave to cool before serving.