Sprinkle strips of sirloin with garlic powder to taste. In a large skillet over medium heat, heat the vegetable oil and brown the seasoned beef strips. Transfer to a slow cooker.Mix bouillon cube with hot water until dissolved, then mix in cornstarch until dissolved. Pour into the slow cooker with meat. Stir in onion, green peppers, stewed tomatoes, soy sauce, sugar, and salt. Cover, and cook on High for 3 to 4 hours, or on Low for 6 to 8 hours.