Preheat oven to 400 degrees F.
Separate the eggs and set aside 1 egg white for another use. You'll be using 2 whites and all 3 yolks.
In one bowl, whisk together the sugar, lemon zest, flour, melted butter, salt and lemon juice.
In a mixing bowl, using an electric mixer, beat the two egg whites until stiff peaks form. Set aside.
In another bowl, beat the egg yolks, then gradually beat in the milk, beating until well mixed. Beat in the sugar mixture, then use a large scraper to fold in the egg whites. You might have to fold a little more energetically than usual to get them blended in. A few lumps here and there should be okay since everything rises to the top anyway.
Pour mixture into the crust. Set it on a rimmed baking sheet.
Bake for 10 minutes at 400 degrees F. Reduce oven temperature to 325 degrees F. and continue baking for 30 minutes or until top is lighly browned and center is just a little bit jiggly.
Let cool to room temperature and then chill until ready to serve.