Mix condensed milk, sour cream, lime juice, and lime zest. Mix well and pour into pie crust.
Bake in preheated oven for 8 minutes, taking care not to brown! Chill pie for at least two hours before serving.
I have found it easiest making it to these quantities, as it uses one can of condensed milk. It makes one very well filled pie crust and leaves a little mix to spare. I can, however, easily make three pies with the double quantity.
Key Limes are difficult to find outside of Florida. In the US you may be able to find Nellie & Joe's Key West Lime Juice, which is an excellent bottled Key Lime juice. Alternatively you can use the juice of regular (persian/mexican) limes. Preferably freshly squeezed or else a good bottled product. If you squeeze limes, place them in the microwave at full power until very warm (approx 60 to 90 seconds) and let them cool to luke warm. That way the juice will come out very easily.