Shepherd's Pie

I have a go to Shepherd's Pie recipe that everyone loves.  But I have to include this one, because it is a great way to use up leftover roast. In fact, that is how I found it. John and I really like it, but my kids aren't quite in love with it.  It is particularly easy if you use potato pearls from the cannery, and I am known for skipping the whole sauted onion step as well.  

• 1 small onion, peeled and chopped
• 14 tablespoons butter, divided (1 stick plus 3/4 stick)
• 1 1/2 pounds leftover pot roast, cubed
• Salt and black pepper
• 8 to 10 medium red new potatoes
• 2 cups milk
• 1/2 cup sour cream
• 2 cups instant biscuit mix
• 1 1/2 cups niblet corn or mixed vegetables, prepared according to package directions
• 1 can cream of mushroom soup
• 
  
• Preheat oven to 350 degrees 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.
  Potato Layer:
  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.
  Biscuit Layer:
  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 9 by 9 by 2-inch pan (or any similar casserole dish) withcooking 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 cream of mushroom. 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.