Preheat the oven to 350 degrees F. In a large mixing bowl, combine the sugar, flour, and lemon zest. Whisk in the eggs, 1at a time. Stir in the buttermilk and melted butter. Pour the buttermilk mixture into the pie shell and bake until the top is lightly browned and the center sets, about 25 minutes. Remove from the oven and cool to room temperature. Slice the pie into individual servings. Garnish with the whipped cream and mint.