Sprinkle strips of sirloin with garlic powder to taste. Heat the vegetable oil and brown the seasoned beef strips in a large skillet over medium heat. Transfer to a slow cooker.

Mix hot water with bouillon cube until dissolved, then mix in corn starch until dissolved. Pour in slow cooker with meat. Stir in onion, green peppers, stewed tomatoes, soy sauce, sugar, salt.

Cover, and cook on high for 3 to 4 hours, or on low for 6 to 8 hours.