Make the crust: Pulse the flour and salt in a food processor. Add the butter and shortening and pulse until the mixture looks like coarse meal with pea-size bits of butter. Drizzle in 3 tablespoons ice water and pulse until the dough begins to come together (add more water, 1/2 tablespoon at a time, if necessary). Turn out onto a sheet of plastic wrap and pat into a disk; wrap tightly and refrigerate at least 1 hour and up to 1 day.
Roll out the dough into an 11-inch round on a lightly floured surface. Ease into a 9-inch pie plate. Fold the overhanging dough under itself and crimp the edges with your fingers. Pierce the bottom all over with a fork. Refrigerate the crust until firm, about 30 minutes.
Place a baking sheet on the middle oven rack and preheat to 425 degrees F. Make the filling: Whisk the heavy whipping cream, sugar, flour and vanilla in a bowl. Pour into the chilled crust, dot with the butter and sprinkle the nutmeg on top. Carefully transfer the pie plate to the hot baking sheet and bake 10 minutes. Reduce the oven temperature to 350 degrees F and continue baking until the crust is golden and the filling is bubbly in spots, about 55 more minutes (the center will still jiggle slightly); cover the crust edges with foil if they're browning too quickly. Transfer to a rack and let cool completely. Serve chilled or at room temperature.