Plum-Blackberry Streusel Pie

Put a foil-lined large baking sheet in lower third of oven and preheat oven to 375 °F. Toss plums and blackberries with 1 cup sugar, tapioca, cornstarch, zest, and 1/4 teaspoon salt in a large bowl. Roll out dough into a 13-inch round on a lightly floured surface with a lightly floured rolling pin. Fit into pie plate. Trim excess dough, leaving a 1/2-inch overhang. Fold overhang under and press against rim of pie plate, then crimp decoratively. Chill while making streusel. Stir together oats, flour, and remaining 1/2 cup sugar and 1/2 tsp salt. Blend in butter with your fingertips until mixture forms small clumps. Mound filling in shell. Crumble streusel evenly over filling. Bake pie until streusel is golden and filling is bubbling, 1 1/4 to 1 1/2 hours. Cool completely, 3 to 4 hours.