This dish is a traditional specialty of the East End of the Hamptons. The clams may be steamed open (briefly) instead of shucked. (Steaming is easier.)

FOR THE PIECRUST:
2 cups flour
1⁄2 tsp. salt
1⁄4 tsp. baking powder
8 tbsp. chilled butter, cut into small pieces
2 tbsp. chilled vegetable shortening, cut into
   small pieces

FOR THE FILLING:
3 medium russet potatoes, peeled and quartered
2 cups clam juice
5 strips bacon, cut into 1⁄2" pieces
1 medium yellow onion, peeled and finely chopped
1⁄4 cup chopped fresh parsley leaves
1 tbsp. fresh thyme leaves
3 cups shucked steamer or cherrystone clams, roughly
   chopped
2 tbsp. flour
Salt and freshly ground black pepper
2 tbsp. chilled butter, cut into small pieces

1. 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.

2. 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.

3. Meanwhile, sauté 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 sauté 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.

4. 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.