First place the milk in a saucepan with the grated zest of the lemons. Bring slowly to the boil while separating the eggs into two basins.
Then mix the sugar into the yolks.
Strain the milk through a sieve on to the egg yolks and sugar, and whisk well before returning the mixture to the saucepan. Then stand the pan over a medium heat and stir until the liquid has almost reached boiling point and has thickened to a thinnish custard consistency. Now take the pan from the heat and leave it on one side to cool. Next sprinkle the gelatine into a small cup or basin containing two tablespoons of water.
Leave it for a few minutes to give the gelatine time to soften, then place the cup in 2.5cm of simmering water in a saucepan. Stir occasionally until the gelatine has dissolved and the liquid is absolutely clear. Then strain it into the cooling custard, mix thoroughly and, when the custard is cold, stir in the lemon juice. Now whisk the egg whites until stiff but not dry, and carefully fold them into the custard mixture. Pour the mixture into a 1½ -litre basin, and leave in a cool place to set.
To serve, loosen around the edges with a palette knife and turn out on to a plate.