Why is it that a lemon meringue pie is such an iconic springtime pie? Likely, it's because of the bright, fresh flavors of the lemon curd filling paired with the fluffy topping! Such a simple pairing is also pretty easy to make, especially with an already-baked pie crust!
In a 4 quart saucepan, combine the four whole eggs, four egg yolks, lemon juice, sugar, cornstarch, and salt. Whisk to combine and place over medium-low heat. Continue to whisk until the mixture begins to thicken with a few bubbles forming around the edges, 5-6 minutes. Keep whisking and allow the curd to cook until it is noticeably very thick with large bubbles forming, 4-6 minutes.
Remove the curd from the heat and pour through a sieve into the prepared pie shell. Place plastic wrap directly on the lemon curd filling and place in the refrigerator until chilled, or for up to 24 hours. Once chilled, top with the meringue (recipe following).
For the Swiss meringue
In the bowl of an electric mixer or a glass bowl, combine the reserved four egg whites, cream of tartar, sugar, and salt. Whisk to combine and place over barely simmering water. Continue to whisk until the mixture thickens slightly and reaches 160°F. Remove from the heat and transfer to a stand mixer.
Using the whisk attachment, whisk the meringue on medium speed, increasing to medium-high until the egg whites cool to room temperature, 3-4 minutes. Continue to whisk until the meringue becomes thick and holds a stiff peak, 4-6 minutes.
Once at stiff peaks, whisk in the vanilla extract. Add the meringue to the prepared and chilled lemon pie.
Using a kitchen torch or broiler, brown the meringue and serve immediately.