Preheat the oven to 190C/375F/Gas 5. Roll out the pastry on a floured surface and use to line the greased flan tins. Prick the base of the pastry with a fork. Line the pastry with greaseproof paper, fill with baking beans and bake blind for 10 minutes.
Remove the greaseproof paper and baking beans and brush the pastry with one of the egg yolks. Return the tins to the oven and bake for a further 15 minutes until golden brown.
Place the rhubarb, seven tablespoons of the sugar and the lemon juice and zest in a pan. Cover and cook over a low heat until the rhubarb has softened then mash the fruit with a fork.
Mix the cornflour and water in a bowl to form a smooth paste. Stir into the rhubarb, bring to a boil and stir until thick. Remove the pan from the heat and stir in the remaining two egg yolks. Use a ladle to pour the mixture into the baked pastry cases.
Whisk the egg whites in a large bowl until they form stiff peaks. Gradually add the remaining sugar, whisking between each addition.
Pipe the meringue over the fruit filling and bake for about 20 minutes or until the meringue is golden-brown.