With the rack in the middle position, preheat the oven to 200 °C (400 °F).
In a food processor, combine flour, sugar and baking powder. Add butter and pulse until the mixture has the texture of sand. Add egg and pulse until dough has a grainy texture. With your fingertips, press dough at the bottom and edges of a 23-cm (9-inch) pie plate. Bake for about 22 minutes or until golden brown. Let cool.
In a saucepan, off the heat, combine sugar and cornstarch. Stir in egg yolks, lemon zest and juice, stirring with a whisk. Add water and bring to a boil, stirring constantly. Simmer for one minute and remove from heat. Strain. Add butter and stir until melted.
Pour into crust. Cover with plastic wrap directly on filling and let cool in the refrigerator.
In a large bowl, over a double boiler, off the heat, combine water and cornstarch. Cook, stirring with a whisk until the mixture thickens. Remove the bowl from the double boiler. Stir in sugar and egg whites. Put the bowl back on the double boiler and heat for about 1 minute or until sugar has dissolved. Remove the bowl from the double boiler and beat with an electric mixer until meringue forms semi-firm peaks. Cover pie with meringue and bake in the centre of the oven at 180 °C (350 °F) for about 15 minutes or until meringue is golden brown. Refrigerate for 6 to 8 hours.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.