Preheat the oven to 350°F and set aside a 9-inch pie plate.
Blitz graham crackers in a food processor until they are crushed into fine crumbs. Combine the crumbs, sugar, and melted butter in a medium bowl. Stir until the crumbs are evenly moistened.
12 full sheet graham crackers, 1/3 cup granulated sugar, 6 Tablespoons butter
Pour the crumb mixture into the pie plate. Use the back of a measuring spoon or the bottom of a small drinking glass to press the crust firmly across the bottom and up the sides of the pan. Bake the crust by itself for 8 minutes, just until fragrant and very lightly browned, then remove from the oven and set aside.
While the crust bakes, make the pie filling by whisking together the eggs, sugar, sour cream or crème fraîche, and salt in a large bowl.
4 large eggs, 1 cup granulated sugar, 1/2 cup sour cream or crème fraîche, 1/4 teaspoon kosher salt
Add the lemon zest and juice. Whisk slowly until the mixture is even. Pour filling into the warm crust.
3/4 cup fresh lemon juice, 1 Tablespoon lemon zest
Bake the pie for 25-35 minutes, until the center is mostly set but still wiggles very slightly when you jostle the pie. Remove from the oven and cool to room temperature on a wire rack.
To make the topping, place the cool water in a small saucepan and sprinkle with gelatin. Set aside for about 5 minutes to soften, then warm the mixture over very low heat, just until the gelatin dissolves. Set aside to cool.
2 Tablespoons cool water , 1 teaspoon unflavored gelatin
In large bowl, use an electric mixer fitted with the whisk attachment to beat the cream and sugar until very soft peaks form. Slowly pour in the gelatin mixture and continue beating until the peaks are still soft but hold their shape more.
1 and 1/2 cups heavy whipping cream , 1 Tablespoon granulated sugar
Using a rubber spatula, spread the whipped cream over the pie. Refrigerate until ready to serve, at least 1 hour or, loosely covered, up to 1 day. Slice and serve cold, garnished with more lemon zest or lemon slices, as desired.