Grate and juice the lemons. Mix the juice with the water and half of the sugar in a saucepan, heating until the sugar has completely dissolved. Pour into a bowl, add the limoncello and leave to chill.
Separate the egg yolks from the whites. Beat the yolks with the rest of the sugar until they become white and frothy, then mix in the mascarpone and lemon peel. Beat the egg whites until they make stiff peaks, then add this to the rest of the mixture. Transfer to a piping bag with a smooth, 0.8 in nozzle.
Soak four sponge fingers in the limoncello syrup, then break them up and lay them in the bottom of four glasses. Cover with a third of the cream mixture. Repeat this process twice more, alternating the layers of sponge fingers and cream.
Chill in the fridge for two hours. Garnish with the mint sprigs and slices of lemon or grated peel.