This recipe is part of our meal celebrating winter's tastiest fruit.
Note: You may substitute any available oranges for the blood oranges.
Whisk hot milk into yolks, then return to saucepan. Cook over medium heat, whisking until thick, about 5 minutes.
Remove mixture from heat; stir in vanilla and 1 tablespoon butter. Cool slightly, cover, and chill at least 4 hours.
Make an egg wash by mixing egg with 1 tablespoon water in a small bowl; set aside. Use remaining butter to coat a large, rimless baking sheet.
Dust countertop with flour, and roll out puff pastry to 13" x 15". Cut and reserve 1-inch strips from each of the 4 edges, then place rectangle on baking sheet. Brush border with water, and press a strip of dough along each side. Brush raised border and interior surface with egg wash. Prick base all over with fork; score the border. Refrigerate for 30 minutes.
Preheat oven to 375°. Bake shell for 15 minutes; prick bottom again with a fork. Continue baking until golden and puffed, but not burned, about 10 minutes more. Do not underbake. Use a metal spatula to transfer tart to a cooling rack.
Turn oven down to 350°. Place pistachios on a baking sheet; roast 5 to 10 minutes until crisp. Cool on a plate, then chop. Meanwhile, prepare oranges; cut off peel and pith. Slice between membranes and remove orange segments.
Spread chilled pastry cream evenly over base of tart. Dry orange segments on paper towels and arrange in rows on top. In a small, heavy saucepan, combine 6 tablespoons sugar, 4 tablespoons water, and Grand Marnier. Bring to a boil, stirring constantly, and reduce to a thick, bubbly glaze; be careful not to burn sugar. Pour mixture over oranges (it will harden as a crunchy caramel); sprinkle with pistachios. Serve at room temperature. Do not refrigerate.