The first step I take when preparing this pie (assuming a pie crust has been made and frozen) is to take the eggs out of the refrigerator and separate them. They're easier to separate when cold.
Then I cover the whites and let them sit at room temperature while I bake the crust and get the other ingredients ready. Room temperature egg whites will be easier to whip into a meringue.
- 1 frozen pie crust (see pie crust recipe for instructions to make your own)
- 3 1/2 to 4 cups chopped rhubarb stalks (about 1 lb)
- 3/4 cup white granulated sugar
- 1 Tbsp orange zest
- 1/4 teaspoon cinnamon
- 1/4 teaspoon ground ginger
- 3 Tbsp instant tapioca or 2 Tbsp tapioca flour/starch (can sub 2 Tbsp corn starch)
- 1 Tbsp cornstarch
- 1/3 cup cold water
- 1/4 teaspoon cream of tartar (can use vinegar instead of cream of tartar, see method instructions)
- 1/2 cup white granulated sugar
- 4 large egg whites (room temperature)
- 1/2 teaspoon vanilla extract
1 Pre-bake the pie shell: Heat the oven to 350°F. Line the frozen pie shell with aluminum foil. Fill two-thirds of the way with pie weights (dry beans work well for this).
Bake for 20 minutes. Remove the crust from the oven. Lift up the edges of the aluminum foil to remove the foil and pie weights from the crust. Poke the bottom of the crust all over with the tines of a fork.
Return the pie crust to the oven, bake for an additional 10 to 15 minutes, or until it is nicely and lightly browned all over. Remove from oven and set aside.
2 Prepare the filling: Gently mix the cut rhubarb, sugar, orange zest, cinnamon, and ground ginger in a medium saucepan. Let sit for 15 minutes so the sugar macerates the rhubarb and liquid is released.
Put the pan on the stove on medium heat. Stir in the instant tapioca or tapioca starch. (By the way, if you are using instant tapioca and you don't like seeing anything that might resemble tapioca, just run it through a blender to pulverize it into tapioca flour.)
Slowly heat up the rhubarb until steamy. If you heat the rhubarb too quickly, the pieces will disintegrate into mush. The mush will still work in the pie, and taste good, but will look like pink oatmeal. You want to gently cook the rhubarb until it is a little tender, but not yet falling apart.
3 Preheat oven to 325°F.
4 Start making the meringue topping: While the rhubarb is cooking, get started on the meringue. In a small bowl, whisk together the sugar and cream of tartar and set aside.
(Note that the cream of tartar is not absolutely required, but as a dry acid, it will help the meringue keep its structure. If you don't have cream of tartar you can alternatively support the meringue by adding a teaspoon of white vinegar to the egg whites when you start to whip them.)
In a small saucepan, whisk the cornstarch and cold water together until the cornstarch is dissolved. Heat on the stovetop and stir with a whisk until the mixture gets bubbly and forms a gel. Remove from heat.
5 Beat egg whites, add sugar and cream of tartar mixture, add cornstarch mixture Make sure that the egg whites you are using have absolutely no pieces of egg yolk or shell in them. Make sure that your mixer bowl for whipping the egg whites is spotlessly clean.
Any residual fat from another baking project in the bowl will make it difficult for the egg whites to whip up into a meringue.
Place the egg whites and vanilla in the bowl of an electric mixer. Start the speed on low and gradually increase the speed to medium.
Once the egg whites are frothy, slowly add the sugar and cream of tartar mixture, one tablespoon at a time. Beat until the sugar is incorporated an the mixture forms soft peaks.
Then add the cornstarch mixture, one tablespoon at a time. Increase the mixer speed to high, and beat until the mixture forms stiff peaks. Be careful not to over beat.
6 Fill pie shell with rhubarb filling: The filling needs to be piping hot when the meringue goes on it, so if it has been allowed to cool, reheat it until it is steamy.
Pour the rhubarb mixture into the pre-baked pie shell, spreading it evenly along the bottom of the shell.
7 Spread meringue mixture on top of rhubarb filling: Use a rubber spatula and immediately spread the meringue mixture evenly around the edge and then the center of the pie to keep it from sinking into the filling.
Make sure the meringue attaches to the pie crust to prevent shrinking. Use the back of a spoon to create peaks all over the meringue.
8 Bake: Bake the pie at 325°F until the meringue is golden brown, about 20 minutes. Transfer to a wire rack and let cool to room temperature. The pie can be refrigerated, but it is best served the same day.