Mix dry ingredients together until it resembles breadcrumbs. Add egg and mix to a paste. Wrap in cling film and refrigerate for min. 6 hours before using. Blind bake pastry in large pie dish or individual tart tins.
Blend cornflour, sugar, zest, juice and water into a saucepan. Cook over med heat until mixture boils and thickens stirring constantly. Take off the heat and stir in yolks and butter. When cool place in blind baked case.
Whisk whites till soft peaks form. Then start adding sugar slowly until all has been added and mix until meringue is thick and glossy. Place on top of tart and bake at 190°C till brown 10 – 15 mins.