Preheat oven to 350 degrees. Bake the crust for about 10 minutes. These crusts are fully cooked, but baking them toasts the graham crumbs and makes them more flavorful and sturdy.
Meanwhile, whisk the condensed milk, eggs, and key lime juice to make the filling. Pour this into the crust and bake for 20-25 minutes. The filling should look set and wiggly, but not loose or watery. Cool on the counter briefly, then chill in the refrigerator for at least 2 hours.
To finish the pie, combine the sour cream and sugar. Spread this mixture evenly over the pie, then garnish with lime zest. Slice and enjoy!
