Ground Turkey Shepherds Pie

Name: Ground Turkey Shepherd's Pie
Ingredients: 1 tablespoon plus 1 teaspoon extra-virgin olive oil; 1 large onion, finely chopped (about 1 ½ cups); 3 celery stalks, finely chopped; 2 medium carrots, finely chopped; 2 garlic cloves, minced; 2 pounds lean ground turkey; Coarse salt and freshly ground pepper; 1 tablespoon fresh thyme; ¾ teaspoon chipotle or regular chili powder; 2 tablespoons plus 1 ½ teaspoons cornstarch; 6 ounces frozen peas (about 1 ½ cups); 1 ½ pounds russet potatoes, peeled and cut into 1-inch pieces; 3 parsnips (6 ounces), peeled and cut into ½-inch pieces; 1 cup plain fat-free yogurt