Mix together the rhubarb, sugar, slices and lemon juice in a bowl.
Grease a cupcake tray with some margarine (not too much).
Cut the pastry into 12cm squares (around 5 layers of sheet deep), and place in 4 spaced out pockets, coat the edges with melted margarine to keep it from drying out (glaze the tips of the pastry corners with maple syrup if desired).
Mix the custard with the purple corn flour and add an optional 1tsp vanilla essence.
Place 1tbsp of the above mixture into the filo cases and top with the rhubarb.
Bake on 180C for 15-18 minutes.
Repeat this methods 3-6 until you've used up all the mixture.