On a large greased baking sheet, preferably 12 x 18 inched, roll out the two 9 inch pie crust. You'll want to overlap them like a ven diagram and use a rolling pin to roll out the middle so you have what looks like a big oval.
Roll and pinch up the edges of the pie crust to make a little wall.
Melt 1 tbsp of butter and brush it over the enter pie crust. Sprinkle the crust edges with 2 tbsp of the sugar.
Bake crust for 15-20 minutes or until edges are golden brown, when crust finishes turn the oven down to 350 F.
Meanwhile, crack and separate egg whites and yolk into two separate bowls.
Pour egg whites into a stand mixer and beat on medium speed with the wisk attachment until foamy. Once foamy, begin adding a 1/2 cup of sugar gradually until gone. Beat until stiff white peaks form, about 10 minutes.
Meanwhile, combine 1 1/2 cups sugar, cornstarch, flour, salt, lemon juice, lemon zest, and milk in a medium saucepan.
Cook over medium-high and stir frequently until mixture comes to a boil. Add in remaining 3 tbsp of butter and wisk until melted.
Once butter has melted, add 1/2 cup of the lemon mixture to the bowl of egg yolks and whip until combined, then wisk egg mixture back into the sugar mixture.
Bring to a boil and continue to cook while stirring constantly until thick.
Your crust should be done by now, before adding your filling, use a spatula to press down any bubbles and air that might have snuck up under the crust during baking.
Pour filling into the middle of the crust and gently work it out the the edges, spread meringue out on top of the lemon filling making sure to go all the way to the edge of the crust to seal the pie.
Bake in the oven for 11-13 minutes, or until meringue is golden brown.
Cool on the pan for 1 hour, then transfer to the fridge to finish cooling for at least two hours before cutting and serving.
Serve chilled to room temp.
It is very important that the filling is hot when you put the meringue on so that the underside of the meringue cooks.