Place the potatoes in a medium saucepan, cover with water, 1 tablespoon of salt as you would pasta water, and bring to a boil for about 16 to 18 minutes, or until tender.
Drain, and return to saucepan, add 1/4 teaspoon salt, sour cream and mash with a potato masher, keep covered.
Preheat oven to 375F with a rack in the center.
Heat a large skillet and add oil over medium heat. Add the shallots, parsley, thyme and sage and cook until almost soft about 2 to 3 minutes.
Add the celery and cook until the vegetables are soft, about 4 to 5 minutes.
Add the frozen mixed vegetables, turkey, 2 cups turkey broth, 1/4 teaspoon salt and black pepper and bring to a boil. Stir and simmer over medium-low heat 8 to 10 minutes, stirring until the flavors meld.
Meanwhile combine 2/3 cup turkey broth with the corn starch and mix well to dissolve. Add to the turkey and cook over medium-low heat, stirring until thickened, about 3 to 4 minutes.
Remove from heat and transfer to an oven safe, deep 9 x 9-inch baking dish or deep round pie dish. Or make them in 4 individual broiler-safe 8 ounce ramekins.
Place the mashed sweet potatoes over the turkey filling.
Bake on a sheet pan 20 minutes, until heated through then finish under a broiler 2-3 minutes for extra color.
Sub Yukon gold or Russet potatoes for sweet potatoes
Switch out the turkey for leftover or rotisserie chicken.
Make a vegetarian pot pie by using vegetable broth, omitting the turkey, and adding extra veggies.
For gluten-free pot pie, double check that your cornstarch is gluten-free or swap it for arrowroot.
Make this dairy-free by using vegan sour cream or yogurt instead.