After one taste, my mom rushed over to get the recipe from me.

Preheat your oven to 375°F (190°C).
In a medium saucepan, combine the rhubarb, 1 1/4 cups sugar, flour, and salt. Cook over medium heat until the rhubarb is tender and the mixture is thickened, about 10 minutes.
Remove from heat and stir in the butter and vanilla extract. Let the mixture cool slightly.
Whisk the egg yolks in a small bowl, then gradually add them to the rhubarb mixture, stirring constantly.
Pour the rhubarb filling into the pre-baked pie crust.
In a clean bowl, beat the egg whites with the cream of tartar until soft peaks form. Gradually add the 6 tablespoons of sugar, continuing to beat until stiff peaks form.
Spread the meringue over the rhubarb filling, sealing the edges to the crust.
Bake in the preheated oven for 10-12 minutes, or until the meringue is golden brown.
Let the pie cool completely before serving.
Variations & Tips
For a twist, you can add strawberries to the rhubarb filling for a sweet-tart flavor. If you're short on time, use a store-bought pie crust to save a step. For a more rustic look, you can skip the meringue and top the pie with a crumble made of oats, brown sugar, and butter. If you're not a fan of meringue, a lattice crust can also be a beautiful and delicious alternative.