Step 1

For the Crust: Preheat oven to 350°F. Spread walnuts on a rimmed baking sheet in a single layer. Toast until fragrant, 12 to 14 minutes. Let cool slightly, then finely chop. In a large bowl (or a mixer fitted with the paddle attachment), whisk together butter, egg yolk, sugar, and salt until light and fluffy. Stir in flour and walnuts to combine, then knead mixture with your hands a few times to form a soft, pliable dough.

Step 2

Press dough evenly into bottom and up sides of a standard 9-inch pie dish. Prick at 1-inch intervals with the tines of a fork; refrigerate until firm, about 20 minutes or, covered, up to 1 day.

Step 3

Bake crust 15 minutes; if it's puffing in places, flatten it with the bottom of a dry measuring cup. Continue baking until crust is golden brown and set, 10 to 12 minutes more. Let cool on a wire rack.

Step 4

For the Filling: Meanwhile, in a medium saucepan, whisk together sugar, cornstarch, and salt to combine. Whisk in juice, 1/3 cup water, and egg yolks until smooth. Cook over medium-high heat, whisking, until mixture comes to a boil and thickens. Boil 30 seconds; remove from heat. Stir in orange zest, lemon zest and juice, and butter until smooth. Immediately pour into baked crust; smooth top. Let cool slightly, then refrigerate until cold and set firmly, at least 4 hours or, loosely covered (once cold), up to 1 day. Cut into wedges and serve cold, with whipped cream.