In a Dutch oven, melt the butter and brown the roast. Remove the roast and saute the onion in the remaining butter until light brown. Stir in the sour cream, 2 cups of chicken stock and the seasonings. Add the roast to the pot and baste with the sauce. Cover and simmer for 2-3 hours or until fork-tender. Add the rest of the chicken stock as needed. Serve with the sauce.