Preheat the oven:
Adjust the oven rack to the center position and preheat to 350°F / 180°C / gas mark 4.
Prepare the crust:
In a mixing bowl, combine the graham cracker crumbs, sugar, and melted butter. Stir until the mixture is well combined and moistened.
Fill the pie pan:
Press the crumb mixture firmly into the bottom and up the sides of a 9-inch / 23 cm deep-dish pie pan. Set aside.
Make the filling:
In a large bowl, whisk together the sweetened condensed milk, egg yolks, and lemon zest. Add the lemon juice and whisk vigorously until fully combined and the mixture thickens.
Bake the pie:
Pour the lemon filling into the prepared crust. Bake for 25 to 30 minutes, until the edges are set and the center is only slightly jiggly.
Cool and refrigerate:
Let the pie cool on a wire rack. Once cool, cover and refrigerate for at least 3 hours, ideally 5-6 hours or overnight, until firm.
Garnish:
Before serving, garnish with whipped cream, lemon slices, a sprinkle of lemon zest, and sliced almonds if desired.
Serve and store:
Slice the pie into 10-12 pieces and serve cold. Leftovers can be stored in the refrigerator for up to 5 days.