In a medium bowl, combine mustard powder, cornstarch, salt, pepper and Carne Asada.
Slice steaks into strips, this is easiest if the meat is still partially frozen. Add meat strips to mustard powder mixture. Stir to completely coat.
Veggie spray generously, a non stick frypan. Over high heat, add coated meat, the meat should sizzle when added. Brown meat on all sides, about 4 or 5 minutes; reduce heat to medium and add onions, mushrooms, carrot and celery. Cook stirring occasionally until onion and mushrooms are soft, about 5 minutes. Add stewed tomatoes, stir to mix, bring to boil, reduce heat to simmer.
Cover and simmer for one hour or until meat is fork tender. If the mixture becomes dry add water as required.