Shepherd's Pie from Leftover Beef Roast and Mashed Potatoes

1 small onion, peeled and chopped 14 tablespoons butter, (1 stick plus 3/4 stick) divided 1 1/2 lbs leftover pot roast, cubed 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