Preheat oven to 350°F.
Pot Roast Layer:.
In a large skillet over medium-high heat, saute onion in 2 tablespoons butter. Add pot roast and cook until browned. Season with salt and pepper, to taste. Set aside.
Peel and slice potatoes 1/4-inch thick. Place in a saucepan, cover with cold water, and bring to a boil. Cook for approximately 15 minutes or until fork tender. Drain potatoes and transfer to a mixing bowl. Whip potatoes with an electric mixer, mixing until moderately smooth. Don't over beat them; a few lumps are nice. Add 1/2 cup heated milk, 1 stick butter, and sour cream. Season with salt and pepper, to taste. Whip again until mixed. Adjust thickness by adding more milk, if desired.
Combine biscuit mix and 1 1/2 cups of milk. The mix should be thinner than that of normal biscuit mix but not runny.
Spray a 9x9x2-inch pan (or any similar casserole dish) with cooking spray. Layer mashed potatoes halfway up the sides of the dish. Next, spread a layer of corn or mixed vegetables on top of the potatoes. Then add the layer of pot roast and onions with gravy. Pour the biscuit mix over the meat. 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.