PREHEAT the oven to 350° F (175° C).

SELECT a fresh lemon with healthy-looking skin. Scrub the lemon(s) with a vegetable scrubber. Cut off the end of the lemon with the pedicel (the nub where the lemon used to connect with the tree). Cut off the other end of the lemon if the peel seems especially thick and pointed.

WEIGH the lemon with a kitchen scale. You need 4 to 6 ounces. If the lemon weighs less than this, use an additional lemon. If you don't have a kitchen scale, use 1 large or 2 medium-sized lemons. Cut the lemon in quarters and remove all the seeds.

POUR the sweetened condensed milk and the lemon pieces into a heavy-duty blender (like the Vitamix) or a food processor.

START on low speed to pull everything together. Increase the speed to high, and puree lemon chunks and milk for 30 seconds, or until it is completely smooth.

MELT the butter in the microwave. About 20 seconds on full power should melt it. Add additional time if needed. Cool for a minute or two. Separate the egg yolks.

EMPTY the lemon puree into a medium bowl. Add the eggs yolks, melted butter, vanilla extract, and salt. Blend with an electric mixer on low speed for 30 to 45 seconds. Blend until the eggs are incorporated into the puree and the lemon cream batter is smooth.

POUR the pie filling into the premade graham cracker crust. Place the pie tin on a cookie sheet to catch any spills as it bakes.

BAKE in the oven for 25 to 30 minutes. The edges will be solid when the pie is done, although the center will still look slightly jiggly.

REMOVE the pie from the oven. Cool it on a cooling rack for 1 hour until it reaches room temperature.

COVER the pie with plastic wrap and refrigerate for 2 hours.

GARNISH with fresh berries, dust with powdered sugar, or serve with whipped cream.