Heat oil in a large Dutch oven over medium-high heat. Place steaks into the pot one at a time, quickly browning on each side. Remove steaks, and set aside. Add the onion to the pot, and cook over medium-low heat until wilted, about 3 minutes. Return the steaks to the pot, and add the tomatoes, mushrooms, celery and green pepper. Season with oregano and add the bay leaf.
Cover the pot, and cook over medium-low heat for about 1 1/2 hours, or until meat is fork tender. If you wish to thicken the gravy, remove the meat to a plate. Stir together the cornstarch and water; stir into the pan juices. Let simmer for about 2 minutes to thicken. Return the beef to the pot.
