For the curd, place rhubarb in a saucepan. Add 55g (1/4 cup) of the sugar and water. Cover and cook over low heat, stirring occasionally, for 10 minutes or until rhubarb collapses. Use a stick blender to blend to a smooth puree. Add egg yolks, butter and 1/2 cup of the remaining sugar to the rhubarb puree. Cook, stirring, over low heat for 15 minutes or until mixture thickens. Set aside to cool.
Process the biscuits in a food processor until finely crushed. Add melted butter. Process until combined. Divide among four 8cm fluted tart tins with removable bases. Press evenly over base and sides. Place in the fridge for 30 minutes to set. Spoon cooled curd among tartlet cases.
Use electric beaters to beat egg whites in a bowl until soft peaks form. Gradually add remaining sugar and beat until completely dissolved. Place in a piping bag fitted with a 1.5cm plain nozzle. Pipe meringue over curd. Use a cook’s blowtorch to caramelise the meringue. Top with roasted rhubarb to serve.