Sprinkle beef sirloin strips with garlic powder. Heat vegetable oil in a large skillet over medium heat and sear beef strips, about 5 minutes per side. Transfer to a slow cooker.
Mix bouillon cube with hot water in a separate container until dissolved, then mix in cornstarch until dissolved. Pour into the slow cooker with beef strips. 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.