Preheat oven to 450 degrees F. Line the frozen crust with heavy duty foil (or two sheets of regular foil) and fill with dried beans, rice, or pie weights. Bake 8 minutes, remove weights and foil, bake another 5-7 minutes. Cool completely.
To a heavy bottomed saucepan, add sugar and cornstarch. Whisk to combine and slowly whisk in milk and lemon juice. Heat over MED-HIGH heat, stirring often, until mixture is thickened and bubbly. Reduce heat down to LOW, and cook 2 minutes longer, stirring very often. Remove from heat.
Lightly beat eggs in a small mixing bowl. Whisking continually, stir in a small amount of the hot lemon mixture (no need to measure), then return mixture to the hot saucepan.
Heat pan over MED heat until it comes to a gentle boil, and continue to cook for 2 minutes. Remove from heat again and stir in cubed butter and lemon zest. Continue to cool, but don't stir.
Cool mixture for about 30-40 minutes, then fold in sour cream until no white streaks remain. Spoon mixture into the prepared pie crust and spread out evenly.
Cool in the refrigerator for 4 hours (or more). To serve, top with optional whipped cream and slice.