Pre-heat your oven to 200 C / 400 F / Gas Mark 6, and spread the Caster sugar over an oven tray lined with baking paper and heat in the oven for five minutes. Meanwhile, wipe the inside of your mixing bowl with the cut lemon and add the egg whites.
Whisk up to a foam, then carefully remove the sugar from the oven and tip a third into the egg whites continuing to mix constantly ( you may need help if you are using a hand mixer ).
Add the remaining sugar and continue whisking until the mixture has cooled, and is glossy and will hold its shape.
Turn the oven down to its lowest setting. Line a baking tray with non-stick baking paper, and spoon the meringue mix on, remember to leave sufficient gaps as they will increase in size as they dry out.
Place them into the oven and bake until they are crisp on the outside, and sound hollow when tapped on the bottom, depending on their size, this could take four to six hours.
Turn the oven off and leave the meringues in there until it has cooled, then immediately transfer to an air-tight container.