Preheat the oven to 350 degrees F (175 degrees C).
Combine condensed milk, key lime juice, sour cream, and lime zest in a bowl; mix well. Pour into graham cracker crust; place pie on a baking sheet.
Bake in the preheated oven for 5 to 8 minutes, until tiny pinhole bubbles burst on the surface of the pie. The filling will be wobbly but not browned.
Transfer pie to a wire rack to cool, then thoroughly chill pie in the refrigerator before serving, about 1 hour. Garnish with lime slices and whipped cream, if desired.