In a large sauté pan over medium heat, sauté the onions in 2 tablespoons of the butter. Add leftover turkey and sauté until hot. Add the gravy and stir well. Season with salt and freshly whacked black peppercorns to taste. Set aside.
Put the potatoes in a large saucepan, cover with cold water, and bring to a boil. Cook until fork tender, about 15 minutes. Drain potatoes well and transfer to a mixing bowl. Whip potatoes with an electric mixer, mixing until moderately smooth. Add ½ cup of the milk, heated, 1 stick butter, and sour cream. Season with salt and freshly whacked black peppercorns to taste. Whip again until mixed. Stir in the chopped fresh parsley. Adjust thickness by adding more milk, if desired. You can use leftover mashed potatoes but freshly made is so much better.
Combine Bisquick and 1 1/2 cups of milk. Add more milk if necessary. It should not be runny but it should be thinner than usual.
Spray a 9 x 9 by 2-inch casserole dish with cooking spray. Layer mashed potatoes on the bottom and halfway up the sides of the dish. Next, spread a layer of vegetables on top of the potatoes. Then add the layer of turkey, onions and gravy. Melt 4 tablespoons of butter and drizzle over top. Bake for approximately 35 to 45 minutes or until top is golden brown. Let stand for a few minutes before serving. Heat the remaining leftover gravy until very hot and serve with the pie.