In a large bowl, whisk together flour and salt.
Cut in butter and shortening using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Gradually add ice water, one tablespoon at a time, mixing until the dough just comes together.
Form the dough into a disc, wrap in plastic wrap, and chill for at least 30 minutes.
On a lightly floured surface, roll out the dough to a 12-inch circle.
Transfer to a 9-inch pie plate, trim edges, and crimp.
Prick the bottom of the crust with a fork.
Line the crust with parchment paper and fill with pie weights or dried beans.
Bake in a preheated oven at 375°F (190°C) for 15 minutes.
Remove the parchment paper and pie weights, and bake for another 5-10 minutes, or until the crust is lightly golden brown.
Let cool completely before filling.
In a medium saucepan, whisk together sugar, cornstarch, and salt.
Gradually whisk in cold water until smooth.
Cook over medium heat, stirring constantly, until the mixture thickens and comes to a boil.
Reduce heat to low and cook for 1 minute, stirring constantly.
Remove from heat.
In a small bowl, whisk a few tablespoons of the hot mixture into the beaten egg yolks to temper them.
Pour the tempered egg yolks into the saucepan and whisk constantly until combined.
Return the saucepan to medium heat and cook for 1 minute, stirring constantly.
Remove from heat and stir in butter, lemon juice, and lemon zest.
Pour the lemon filling into the cooled, pre-baked pie crust.
In a clean, dry metal or glass bowl, beat egg whites with cream of tartar using an electric mixer until soft peaks form.
Gradually add sugar, one tablespoon at a time, beating continuously until stiff, glossy peaks form. The meringue should hold its shape without drooping.
Immediately spread the meringue over the hot lemon filling, making sure to seal it to the edges of the crust to prevent shrinking.
Create swirls or peaks in the meringue using the back of a spoon or a spatula.
Bake in a preheated oven at 350°F (175°C) for 12-15 minutes, or until the meringue is golden brown.
Remove from oven and let cool completely at room temperature before slicing and serving. This is crucial for the meringue to set properly and prevent weeping.