If starting with frozen rhubarb, you will want to thaw the rhubarb completely in a colander set over a bowl so the excess moisture will drip off. Even after thawing, press lightly down on the rhubarb in the colander to encourage a bit more of the liquid to leave the rhubarb.
Preheat the oven to 350F (non-convection/not fan-assisted).
Place the thawed pie shell onto a baking sheet and set aside.
For the filling: In a large bowl, whisk together the white sugar, flour and salt. Add the egg yolks and vanilla and stir together into a thick batter. Add the rhubarb and fold into the filling. Pour the filling into the pie shell.
Bake the pie in the preheated oven for 50-55 minutes or until the centre of the pie is set.
Tip! Test the firmness of the egg whites by dipping the beaters into the egg whites, pulling out and inverting. Look at the dollop of egg whites at the end of the beater. If they start to droop, you should beat them a bit longer. Stiff peaks will hold their shape and not droop.
For the meringue: When the pie is almost done, using a stand mixer with the whisk attachment or in a large bowl with an electric beater, beat the egg whites on medium speed until foamy (like a bubble bath). With the mixer still on medium, gradually add the white sugar until it is all added. Increase the mixer speed a notch and continue beating for a minute or so. Increase the mixer speed again and continue beating for another minute. Finally, increase the mixer to high speed and beat until the egg whites form very stiff peaks. This whole process will probably take about 5 minutes or a bit more.
When the pie is baked, remove it from the oven and immediately spoon the whipped egg whites over the top of the pie. Use the back of a spoon to spread the meringue towards the crust of the pie, making sure to overlap the crust with the meringue and sealing the meringue to the crust by pressing it down gently against the crust with the spoon. Make sure there are no gaps around the edge of the pie where you can see the filling. You want it to be well sealed by the meringue. This will prevent the meringue from shrinking. Finally, use the spoon to make big swirls in the meringue.
Return the pie to the oven for 10 minutes or until the meringue is set and the edges of the swirls are golden.
Enjoy the pie warm or allow it to cool. If the pie will be enjoyed within a couple of hours, you can leave it at room temperature; otherwise, transfer it to the refrigerator to store. Enjoy the pie cold from the fridge or at room temperature.
Notes
Note 1: This vintage pie recipe is generously sweetened. I've never found it to be too sweet, but if you prefer a less sweet rhubarb pie, you can reduce the sugar in the filling by 1/4 cup.Note 2: I like to sneak an extra egg white into the meringue for a lofty and generous meringue topping. You could just use the 3 egg whites left from the filling if you like, though the topping will obviously be thinner than shown here. I would reduce the sugar in the meringue by a Tablespoon or two if using just 3 egg whites.Note 3: I have found that frozen rhubarb, even when thawed and drained, will result in a slightly looser filling. It is still lovely and sturdy enough, and not enough reason to avoid using up your frozen rhubarb in this pie, in my opinion.Be sure to read the notes above this recipe card, where you'll find substitution suggestions and step-by-step photos that you might find helpful.