In a large heavy saucepan, mix sugar and cornstarch.
In a small bowl, add eggs and with a fork mix until combined.
Whisk in milk and lemon juice until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir 2 minutes longer. Remove from heat.
Whisk a small amount of hot mixture into egg yolks.
Return all to the pan, whisking constantly. Bring to a gentle boil; cook and stir 2 minutes. Remove from heat.
Stir in butter and grated lemon peel. Cool without stirring.
When curd has cooled to room temperature, stir in sour cream.
Add filling to baked crust. Top with whipped cream. Store in the refrigerator. Yield: 8 servings.
Notes
Pie Crust Instructions:
Prepare pie crust and place it in a nine-inch tart pan. Cover, the tart crust with plastic wrap and place it in the refrigerator for three hours. Rest it overnight if you're splitting this project into two steps.
If you're using a purchased pie crust, no need to refrigerate for three hours, but you'll need to follow the dock and bake procedures in the next steps.
Before starting the lemon tart curd process, remove the tart from the refrigerator and with a fork stab the pastry shell on the bottom and sides. This is called docking. This allows the steam to release while cooking so the pastry doesn't bubble up.
Place a sheet of parchment paper over the shell and fill the shell with beans to weight the paper. Another step to keep the crust from bubbling up while baking. Again, refer to my Step By Step Pie Crust post for more details.