FOR THE PIECRUST:

FOR THE FILLING:

For the piecrust: Mix together flour, salt, and baking powder in a medium bowl. Quickly work butter and shortening into flour mixture with the tips of your fingers until mixture resembles coarse meal. Sprinkle with 8 tbsp. ice water and stir with a fork until just combined. Form into a ball, then flatten into a disk and divide in half. Cover with plastic wrap and refrigerate for at least 30 minutes.
For the filling: Boil potatoes in clam juice in a covered pot over medium heat until soft, about 30 minutes. Drain, lightly mash with a fork, and set aside.
Meanwhile, saute bacon in a skillet over medium-low heat until crisp, about 7 minutes. Transfer bacon to a paper towel. Add onions to bacon fat and saute until golden, about 15 minutes. Mix together bacon, onions, parsley, thyme, clams, flour, and potatoes in a bowl. Season with salt and pepper to taste.
Preheat oven to 400°. Roll out dough on a lightly floured surface into two 12" circles. Fit 1 circle into a 9" deep-dish pie pan. Add clam filling, mounding it in the center. Top with butter. Lay remaining dough over filling. Trim dough, leaving about 1" around edge; fold under and crimp two layers together. Make a little hole in center of pie to allow steam to escape. Bake until golden, 50-55 minutes.