To make the meringue: Put the caster sugar and water into a small saucepan and stir until the sugar has melted, then bring to the boil. Leave the saucepan without stirring, until it reaches a fast boil. Use a sugar or digital thermometer to check the temperature of the sugar syrup — it needs to reach 248F.
Meanwhile, put the egg whites and lemon juice into the bowl of a stand mixer and whisk until it reaches soft peaks.