Thanksgiving might mean that winter is on its way, but it also means citrus season is getting near. This pie will be a vibrant note on your sideboard.
Prepare the Crust: Preheat oven to 375°F. Unwrap chilled pie dough disks from Double-Crust Pie Pastry, and place on a lightly floured surface. Let stand at room temperature until slightly softened, about 5 minutes. Sprinkle dough disks with flour. Roll 1 disk into a 12-inch circle, and carefully fit into a 9-inch deep-dish glass pie plate. Trim dough to cover flat rim of pie plate. Set aside in refrigerator.
Roll remaining dough disk to 1/4-inch thickness. Use a 1-inch leaf-shaped cutter or a paring knife to cut out, free-form, 35 to 40 leaves from pie dough piece. Use back edge of paring knife to score lines in dough leaves to resemble veins.
Brush dough leaves with water, and attach to piecrust edge; gently pressing to adhere. Sprinkle with sugar. Chill 30 minutes.
Cut a square of parchment paper large enough to completely cover pie plate. Crumble parchment, and crush into a tight ball. (This helps soften its texture so it will fit over the unbaked piecrust evenly.) Open ball of crushed parchment sheet, and flatten out. Carefully line pastry with parchment, covering inner pastry well and shielding dough leaves without touching them. Fill with pie weights or dried beans.
Bake piecrust in preheated oven 15 minutes. Remove piecrust from oven, and carefully remove pie weights and parchment paper. Return to oven, and bake until mostly dry and golden, about 10 minutes, loosely covering dough leaves with aluminum foil or parchment paper to prevent overbrowning. Transfer crust to a wire rack, and cool 30 minutes.
Prepare the Filling: Reduce oven temperature to 300°F. Finely grate 1 grapefruit to yield 2 teaspoons zest; set aside.
Slice off stem end and base of each grapefruit, exposing juicy pulp inside. Place 1 grapefruit on a cutting board with flat base down. Cut off a 2-inch-wide strip of peel and white pith, slicing deeply along the curved side to expose juicy pulp inside. Cut another strip from opposite side of fruit, and continue, alternating sides, completely exposing fruit. Discard peel and pith.
Working over a shallow bowl to catch the juice, remove grapefruit segments from pith using a small, sharp knife. Hold fruit in 1 hand and a small knife in the other, slicing along inside wall of 1 segment from outside to center. Slice other side to release juicy segment, and place in a bowl. Continue around the fruit, squeezing core over bowl to capture all the juice. Gently strain segments into bowl, reserving segments and juice separately. You will need 1 cup grapefruit segments and 1/2 cup grapefruit juice. Stir grapefruit zest and salt into grapefruit juice. Reserve any extra segments or juice for another use.
Whisk together eggs and egg white in a large bowl until light, creamy, and smooth. Whisk in cream and melted butter. Stir together sugar, flour, and cornmeal in a medium bowl. Add to egg mixture, in 3 batches, whisking well after each addition. Add grapefruit juice mixture; stir until well combined, smooth, and creamy.
Carefully spoon filling into prepared piecrust. Arrange grapefruit segments on filling, placing gently so they float on top. Place pie on a rimmed baking sheet. Loosely cover dough leaves with aluminum foil to avoid overbrowning.
Bake at 300°F until top is browned and pie filling is set, about 50 minutes, removing foil for last 5 to 10 minutes of baking. Transfer pie to a wire rack, and cool completely, about 2 hours. Serve with sweetened whipped cream.