Add flour, butter and salt into a food processor. Pulse the machine on and off until the mixture is crumbly. With the machine running, slowly add the water until just combined (do not over-mix). Wrap the dough in plastic wrap and flatten into a disc. Chill for at least 2 hours or overnight. (Can be made up to 72 hours in advance).
Allow the dough the rest at room temperature for 5 minutes. On a lightly floured surface, roll the dough to approximately 1/4-inch thin. Carefully place the dough into a shallow pie dish. Place the dish back in the refrigerator and chill for 30 minutes.
Preheat the oven to 400 degrees F. Place the pie dish on a baking sheet. Use a fork to poke holes throughout the bottom of the dough to prevent air bubbles from forming. Line the pie pan with foil and cover with uncooked beans. Bake for 20 minutes, removing the beans and foil for the last 5 minutes. Allow to cool while preparing the other ingredients.
Preheat the oven to 325 degrees F. Place the prepared pie crust on a baking sheet.
Whisk together the sweetened condensed milk, grapefruit juice and eggs.
Pour the custard into the pie shell, leaving at least 1/4 inch around the top.
Carefully place the baking sheet with the pie in the oven and bake for 25 minutes. It will still be wobbly but should look slightly firmer.
Chill in the refrigerator for at least 2 hours or overnight to set the custard. Serve chilled or slightly above room temperature.
This recipe uses a shallow 9-inch x 1-inch pie pan, such as this one. A store-bought pie dough can be used to save time.