Mix together all of the ingredients for the pie crust and knead by hand. Wrap the kneaded dough in plastic wrap and refrigerate for at least 1 hour. Once chilled, take the dough out of the refrigerator, roll it out into a circle about 1/4-inch thick, transfer it to a pie dish, and trim off any excess dough.
Place a piece of parchment paper on top of the dough and pour baking beans on top. Transfer the pie dish to a preheated oven set to 350°F and bake for 45 minutes. Let the pie crust cool completely, then remove the baking beans and parchment paper.
Mix the gelatin powder with 2/3 cup of cold water and let sit for 10 minutes. Heat 3 1/3 cups of water in a pot on the stove and add the sugar. Squeeze the juice of 4 lemons into the pot and bring to a boil. Let the mixture cool to about 140°F and stir in the gelatin mixture. Strain the final mixture through a cheesecloth or very fine strainer, and once it has cooled to about 85°F, pour the liquid over the pie crust and refrigerate the pie dish for 6-8 hours, or preferably overnight.
Beat the egg whites and salt until stiff, gradually incorporating the sugar. Sift the powdered sugar and optional corn starch and gently fold it into the mixture.
Spread the meringue over the transparent pie layer.
Using a culinary torch, lightly brown the surface of the meringue.