Caramelized Onion and Sweet Potato Tart

2 cups all-purpose flour 1/4 cup cold butter 1/3 cup shortening 1/3 cup cold water 1 pinch salt 1 lb yukon gold potato 1 lb sweet potato 3/4 lb parsnip 3[...]

1 teaspoon vegetable oil 1/4 large sweet onion , sliced (Vidalia or Spanish) 3 medium sweet potatoes , peeled and sliced 2 tablespoons packed brown sugar 1/2[...]