For the rhubarb: Combine the rhubarb, sugar and water in a heavy-based saucepan.
Cook over a medium heat, stirring, until the sugar has dissolved and the fruit is very soft, about 12-15 minutes. Remove from the heat and stir in the lemon juice.
Transfer the rhubarb and syrup to a bowl. Cover and chill until cold, about 2 hours.
For the cream: Beat together all the ingredients for the cream in a mixing bowl until smooth and creamy.
To assemble: Break the lady fingers into large pieces, arranging them in the bases of four serving jars.
Soak with a little of the rhubarb syrup. Spoon the mascarpone cream on top and top the cream with the rhubarb fruit.
To serve: Serve immediately with a garnish of pistachios or cover and chill until ready to serve.