In a medium bowl, whisk the flour, sugar and 1/2 tsp. salt. Using your fingers, a fork or a pastry cutter, work the butter into the flour in two batches, until pea-sized pieces form. In a cup, combine the ice water and lemon juice. Sprinkle the liquid over the flour mixture; stir with a fork until fully incorporated and the dough is crumbly. Form the dough into a ball and flatten it into a disk; wrap in plastic. Refrigerate for at least 1 hour or up to 2 days.
Let the dough sit at room temperature for 15 minutes. Unwrap and place on a lightly floured work surface. Lightly sprinkle the dough and a rolling pin with more flour. Roll out the dough into a 14-inch round (about 1/4 inch thick). Sprinkle the top lightly with flour, then drape the dough gently onto the rolling pin; transfer to a 9-inch deep-dish pie pan. Press the dough into the pan; crimp any excess around the edges.
Preheat the oven to 375°. Line the pie shell with parchment and fill it with pie weights or dried beans. Bake until the crust is cooked through and golden, about 35 minutes.
Meanwhile, in a large bowl, whisk the brown sugar, cornstarch, cinnamon, ginger, mustard and remaining 1/2 tsp. salt. In another large bowl, beat the eggs; whisk in the pumpkin, cream and milk to combine. Stir the egg mixture into the sugar mixture until well combined.
Pour the filling into the pie crust and bake until the filling is mostly set and barely wobbly in the center, 55 to 60 minutes. Cover the edges with foil if they are browning too quickly. Transfer to a rack to cool completely, at least 2 hours. For a firmer pie, refrigerate before serving.