For the crust:
2 cups all-purpose flour
14 tablespoons (1¾ sticks) cold unsalted butter, cut into pieces
1 tablespoon cold vegetable shortening
Scant ½ teaspoon salt
Scant ½ teaspoon sugar
1 egg

For the filling:
5 tablespoons unsalted butter
1 cup chopped carrot
1 cup chopped celery
1 cup thinly sliced leeks
1 tablespoon chopped fresh tarragon
1 cup frozen baby green peas
4 tablespoons all-purpose flour
2½ cups chicken broth
1 cup heavy cream
Salt and freshly ground pepper
3 cups leftover roast chicken, cut into ½-inch chunks