This key-lime pie can be topped with either whipped cream or meringue.
For the Crust: Heat oven to 375 degrees. Combine graham-cracker crumbs, butter, and 3 tablespoons sugar in a medium bowl; mix well. Press into a 9-inch pie plate, and bake until lightly browned, about 12 minutes. Remove from oven, and transfer to a wire rack until completely cooled.
For the Filling: Lower oven to 325 degrees. In a medium bowl, gently whisk together condensed milk, egg yolks, key lime juice, and zest. Pour into the prepared, cooled crust.
Return pie to oven, and bake until the center is set but still quivers when the pan is nudged, 15 to 17 minutes. Let cool completely on a wire rack.
For the Whipped Cream Topping: Shortly before serving, combine cream and sugar in the bowl of an electric mixer fitted with a whisk attachment. Whisk on medium speed until soft peaks form, 2 to 3 minutes. Spoon over cooled pie and serve immediately.
For the Meringue Topping: Add topping shortly before serving. In a large bowl, combine egg whites, sugar, and salt. Using an electric mixer, beat on medium-high until stiff peaks form. Pipe or spread meringue on pie, making sure it touches the crust all around. Use a kitchen blowtorch to brown meringue all over.