Melt the butter and add the oil. Add onion, carrot, and celery and cook until they begin to brown. Add flour and brown, stirring all the while. Add bay leaf, thyme, garlic, pepper and tomato. Simmer together for a minute and add the hot beef stock. Bring to a boil and reduce heat. Simmer, skimming from time to time, for an hour or more. Strain and cool before refrigerating. Source: Great Chefs of New Orleans, Tele-record Productions : Box 71112, New Orleans, Louisiana - 1983 : Chef Claude Aubert, Arnauds Restaurant, New Orleans