Preheat the Oven: Preheat the oven to 350°F (180°C / gas mark 4).
Prepare the Pie Crust: On a lightly floured surface, roll the pie dough into a 12-inch circle, turning it a quarter after each roll and adding flour as needed to prevent sticking. Transfer the rolled dough into a 9-inch deep-dish pie pan, smoothing it gently. Trim any excess overhang and flute or crimp the edges if desired.
Par-Bake the Crust: Line the crust with parchment paper and tin foil, filling with pie weights or dried beans. Par-bake the crust for 10 minutes, then remove the weights, foil, and parchment and bake for another 10 minutes.
Cook the Cranberries: In a medium saucepan, combine the cranberries and orange juice. Cook over medium heat until simmering, continuing until most berries burst (about 4-5 minutes).
Puree and Strain Cranberries: Use an immersion blender to puree the cranberry mixture. Strain the puree through a fine-mesh sieve, collecting 2 cups of the smooth cranberry mixture in a bowl.
Make the Filling: In a large bowl, combine the sweetened condensed milk, egg yolks, cranberry mixture, and orange zest. Whisk until smooth and well-combined.
Bake the Pie: Pour the cranberry custard filling into the pre-baked pie crust. Bake for 30-35 minutes, or until the center is mostly set but slightly wobbly. If the crust darkens too much, cover the edges with aluminum foil or a pie shield. Start checking for doneness at 25 minutes, then every 5 minutes.
Cool and Chill: Allow the pie to cool on a wire rack for 2 hours, then refrigerate for at least 4 hours or overnight for best results.
Decorate and Serve: Garnish with whipped cream, cranberries, red currants, sugared rosemary, or snowflake sprinkles if desired.
Storage: Cover any leftovers and store in the refrigerator for up to 4 days.