Combine the 1 cup sugar, tapioca, cinnamon, nutmeg, and salt in a large bowl. Add the huckleberries (if frozen, be sure berries are half-thawed before using) and lemon juice and fold everything together gently to combine well. Let stand 15 minutes. (It’s important to do this to soften the tapioca).
Adjust an oven rack to the lowest position, and set a heavy baking sheet on the rack. Preheat the oven to 450 degrees.
While the filling stands, assemble the pie. On a lightly floured surface, roll the second piece of dough into an 11-inch circle. Spoon the huckleberry filling into the cold pastry-lined pan and dome it slightly. Distribute the butter pieces evenly over the filling.
Use a pastry brush dipped in water to moisten the edges of the bottom pastry and cover with the top pastry. Press edges together firmly. Trim excess top crust to be flush with bottom crust. Fold the double thickness of pastry back on itself to make a standing rim and flute.
Brush the top of the pastry lightly with water—try to avoid making little pools of water—and make 4 to 6 slits for steam to escape. Sprinkle top evenly with 1 tablespoon sugar.
Put the pie onto the heavy baking sheet and bake 25 minutes at 450 degrees. Reduce the oven temperature to 375 degrees, and bake until the pastry is a deep golden brown and the pie juices bubble thickly through the slits, about 50 minutes more.
Cool the pie completely on a wire rack, at least several hours, before cutting and serving. If you cut the pie too soon, the filling will run all over the place. You can hasten the setting of the filling by refrigerating the pie for 3 to 4 hours after it has become warm.