The bold, tart taste of red currants is an intriguing addition to more familiar raspberries in this pie bursting with the flavors of summer.
On a lightly floured work surface, roll out half the dough into a 1/8-inch-thick circle about 12 inches in diameter. Drape dough over a 10-inch pie plate; transfer to refrigerator to chill, about 30 minutes.
Whisk together the egg and milk to make an egg wash; set aside. In a medium bowl, mix together currants, raspberries, tapioca, lemon zest and juice, and granulated sugar. Once fruit is coated, turn out mixture into pie pan. Dot top of filling with butter.
Preheat oven to 425 degrees. Roll out remaining dough to the same size and thickness as chilled dough. Brush rim of chilled dough with egg wash; place other piece of dough on top, and trim to 1/2 inch over edge of pan. Crimp edges; place pie in freezer (about 20 minutes) or refrigerator (about 40 minutes) until firm.
Remove from freezer. Cut a few slits in top. Brush with egg wash, and sprinkle with sanding sugar, if desired. Bake 20 minutes. Reduce heat to 350 degrees. Continue baking until juices are bubbling and crust is golden brown, about 40 minutes more. Let cool before cutting and serving.
Fresh currants are available at gourmet markets from June through August. Choose unpackaged fruits that are full and vibrant.