Heat a casserole pot or a heavy bottomed sauce pan to a medium to high heat. Add the oil and butter and brown the beef. Do it in small batches so it browns instead of stews. Place to one side when all beef is done. In the same pan in goes the onions and garlic. Tie the rosemary and thyme together and put that in as well. Cook for 4-5 mins then add the left over marinade from the beef, beef stock, Worcestershire sauce, mustard, mushrooms and put the beef back in. Cover the pot and allow to simmer for 45mins-1 hour until the meat is tender.