Gently whisk the egg whites until soft peaks form. Place the rest of the ingredients in a pan and heat to 115°C to form a syrup. Start whisking the whites on high speed; this is best done using a stand mixer. Add the syrup while hot, and keep whisking until the meringue becomes cool.
Fold the meringue into the lemon mousse and season with a little salt – if the mix is a little sweet add