Combine flour, salt, and sugar in a large bowl. Cut in butter with a pastry cutter or 2 knives (mixture should resemble coarse cornmeal). Sprinkle with just enough ice water, about 3 tbsp., so dough holds together, then divide in half, form into 2 balls, wrap both in plastic, and refrigerate for at least 1 hour.
Combine raisins, orange juice, and zest in a medium saucepan and simmer over medium heat until raisins soften, about 5 minutes. In a small bowl, mix cornstarch with 2 tbsp. water. Stir cornstarch mixture into simmering raisin mixture. Add allspice, nutmeg, and sugar and cook, stirring constantly, until mixture thickens, about 5 minutes. Remove from heat and stir in lemon juice and walnuts. Set aside to cool completely.
Preheat oven to 350°. Remove dough from refrigerator. Flatten 1 ball into a round disk, then roll into a thin 12" round on a lightly floured surface. Ease into a 9" pie pan.
Roll out remaining dough into a second 12" round. Using a sharp knife or pastry cutter, cut dough into 12 1"-wide strips. Spoon filling into pie shell. Lay 6 pastry strips across filling, then lay remaining strips over them, forming a crosshatched crust. (This dough is too tender to weave into a traditional lattice). Cut away and discard any excess dough. Crimp edges of crust and bake pie until crust is golden and filling bubbly, about 30 minutes. Pie is best at room temperature.