Heat the vegetable oil in a large frypan over medium heat. Add the sliced sirloin and salt, pepper, garlic powder and onion powder to taste. Stir and saute for 5 to 10 minutes, or until meat is well browned.
Add the onion and saute for 5 more minutes. Then stir in the capsicum, tomatoes, soup and milk. Mix this all together well, reduce heat to low and simmer for 10 to 15 minutes, stirring often.