Martha Stewart's Shepard's Pie

For the topping:
1 1/2 pounds potatoes, cleaned and cut into 1-inch pieces
3 parsnips (6 ounces), peeled and cut into 1/2-inch pieces
1 cup plain yogurt
salt and freshly ground pepper

For the filling:
2 tablespoons olive oil
1 1/2 teaspoons salt
1 teaspoon ground black pepper
1 large onions, finely chopped
3 celery stalks, finely chopped
2 medium carrots, finely chopped
2 cloves garlic, minced
2 lb. ground turkey
2 tablespoons all-purpose flour
1 tablespoon tomato paste
1 teaspoon thyme leaves (dried, or chopped if fresh)
1 tsp. chili powder
2T plus 1 1/2 tsp. cornstarch
1 bag of frozen peas

Make the topping: Place potatoes and parsnips in a medium saucepan, cover with water by 2 inches, and bring to a simmer. Cook until fork tender, about 20 minutes. Drain, return to saucepan, add yogurt and 3/4 teaspoon salt. Mash and mix together; season with pepper. reserve 2 C of the cooking liquid.

Make the filling: Heat 2 tablespoons of oil in a 12-inch skillet over medium heat. Add onions,celery, garlic and carrots, cook until vegetables are tender, 10-12 minutes.
Add turkey; cook breaking up any large pieces, for 5 minutes. Stir in the seasonings. Whisk together the reserved liquid and the cornstarch, add to the turkey. Boil for 1 minute. Dump contents into a crockpot.

Place the potatoes on top. Cover and cook on low for 4 hours.