In a small bowl, combine the water and sour cream; set aside.

In a large mixing bowl, stir the flour, sugar and salt together. Scatter the shortening over the flour, then use a cheese grater to grate the butter on top. Using a fork or a pastry blender, cut the mixture together until it resembles coarse crumbs. Add the sour cream mixture and use a stiff rubber spatula to stir and press the dough together until it forms one big mound. If dough is still too dry and doesn’t hold together, add another tablespoon of water.

Turn the dough onto a piece of plastic wrap and flatten into a 5-inch disk. Wrap tightly and refrigerate for at least one hour.

Once chilled, roll the dough out into a 12-inch circle and fit into a greased 9-inch pie plate. Trim, fold, and crimp the edge of the dough. Cover with plastic wrap and place in the freezer for 15 minutes until chilled and firm.

Preheat the oven to 400ºF. To blind-bake, I like to press a sheet of aluminum foil into the pie pan since it gets into the crevices better than parchment paper. I then pour pie weights (or dried beans or rice) on top of the foil. Place pie plate onto a baking sheet and bake until the edges of the crust are starting to brown, about 15-16 minutes. Remove pie from the oven and carefully lift the aluminum foil (with the dried beans or rice) out of the pie. Return the pie crust to the oven and bake until the bottom crust is golden brown, about 10-13 minutes longer. Remove and cool completely.

In a medium saucepan, whisk the sugar, cornstarch, and salt together. Add the egg yolks, zest, and 1/4 cup of the grapefruit juice, whisking until combined. Pour in the rest of the grapefruit juice, and whisk until smooth.

Cook mixture over medium heat, whisking constantly until the filling has completely thickened and large bubbles begin to form on the surface, about 7-8 minutes.

Place the egg whites into the bowl of a stand mixer fitted with a whisk attachment (or large mixing bowl with a beater), and whisk on high speed until stiff peaks form.

Pour the sugar and water into a small saucepan, and cook over medium heat, without stirring, until it reaches 240ºF. This usually takes about 5-6 minutes from the time it begins to boil. Immediately pour the hot sugar syrup in a slow, steady stream into the whipped egg whites, with the mixer running on medium speed; try to avoid hitting the whisk attachment.

Once all the sugar syrup has been added, add the cream of tartar, lemon juice, and vanilla. Continue beating the meringue on high speed for 7-9 minutes, or until it’s cooled to room temperature and glossy peaks form.

Using a spatula, spread the meringue evenly on top of the chilled pie filling. Use a kitchen torch to toast the meringue, or place the pie under the broiler, watching closely so it does not burn.

Serve within 6 hours, keeping chilled until ready to serve.