Preheat oven to 325 degrees F. On the stovetop, heat the oil in a large Dutch oven or deep-sided roasting pan set over medium-high heat. Salt and pepper the roast, brown on all sides, and then remove and reserve. Add onions, mushrooms and garlic to pan and cook until just tender, about 5 minutes. Sprinkle in flour and mix to combine. Slowly whisk in stock. Add remaining ingredients, except parsley and sour cream, and bring to a simmer. Return roast to the pot. Cover and cook in the oven 2 hours, turning once. Remove roast from pot, cover, and let rest 10 minutes before slicing. Skim excess fat from sauce, and then mix in parsley and sour cream. Season to taste. Ladle sauce it a gravy boat and serve it alongside the meat.