Slice the rhubarb and put the pieces in a saucepan with the ginger and 2 tablespoons of the water. Cover and simmer for 5 minutes until just tender and still bright pink. Mash or purée.
Put the remaining water in a small bowl and sprinkle over the gelatine, making sure that all the powder is absorbed by the water. Set aside to soak for 5 minutes.
Whisk the egg yolks and sweetener until just mixed. Pour the milk into a small saucepan and bring just to the boil. Gradually whisk the milk into the egg yolks, then pour the mixture back into the saucepan. Slowly bring the custard almost to the boil, stirring continuously, until it coats the back of the spoon. Do not allow the custard to boil or the eggs will curdle.
Take the pan off the heat and stir in the gelatine until it has dissolved. Pour into a bowl, stir in the cooked rhubarb and leave to cool.
Whisk the egg whites until stiff, moist peaks form. Fold the crème fraîche and a few drops of colouring, if used, into the cooled custard, then fold in the whisked whites. Spoon into six glasses and chill for 4 hours until lightly set. Decorate with orange rind just before serving.