Preheat your oven to 375°F (190°C). Thaw the store-bought crust for 10 minutes, then prick the bottom with a fork. Brush the crust with melted butter and sprinkle with sugar. Blind bake for 8 to 10 minutes until set but not fully golden, then remove and let cool.

Roll lemons firmly on the counter, cut in half, and juice to obtain 1/2 cup of fresh juice. Strain through a fine-mesh sieve to remove pulp and seeds.

Zest lemons using a microplane to collect 2 tablespoons of lemon zest, avoiding the bitter white pith. Set aside.

In a large bowl, whisk together sweetened condensed milk, fresh lemon juice, and lemon zest until smooth. Add the egg yolks and whisk for 2 minutes until pale and fully combined.

Optionally, make meringue by whipping egg whites with cream of tartar on medium speed until soft peaks form. Gradually add granulated sugar, one tablespoon at a time, and beat until stiff glossy peaks form.

Pour the lemon filling into the pre-baked crust. If using meringue, spread it over the filling, sealing to the edges. Bake at 375°F (190°C) for 12 to 15 minutes until meringue peaks are golden brown.

Remove the pie from the oven and cool on the counter for at least 30 minutes without moving to prevent cracking or weeping.

Transfer the pie to the refrigerator and chill for at least 4 hours or overnight before slicing and serving.