With the rack in the middle position, preheat the oven to 100 °C (200 °F). Line a baking sheet with parchment paper.
In a bowl, beat the egg white and cream of tartar with an electric mixer until soft peaks form. Gradually add the sugar and beat until stiff peaks form. Spread the meringue on the baking sheet into a disc the same size as the pie. Bake for about 3 hours or until the meringue is dry. Let cool completely.
With the rack in the middle position, preheat the oven to 180 °C (350 °F).
In a bowl, combine the cookie crumbs and butter. Press into a 23-cm (9-inch) pie plate. Bake for about 10 minutes or until the crust is golden brown. Let cool.
In a saucepan, off the heat, combine the sugar and cornstarch. Add the grapefruit zest and juice, water, and egg yolks. Combine with a whisk.
Bring to a boil, stirring constantly. Remove from the heat, add the butter and stir until melted. Strain through a sieve over the crust. Place plastic wrap directly over the filling and refrigerate for about 2 hours or until the pie is cold. Just before serving, remove the plastic wrap. Break the meringue into large pieces and place over the pie. Serve cold. Store in the refrigerator.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.