Slice the beef to a thickness of 0.5cm and marinate it for 30 minutes.
Heat a pan over high heat until it starts smoking. Add the oil and the beef. Pan-fry them for 1 minute on each side. Set them aside.
Cook the spaghetti until al dente. Drain pasta (reserve ½ cup of the cooking water) and set aside.
Heat a pan over medium heat. Add the remaining oil. Stir-fry the bell peppers and onions until slightly softened. Add the beef and pasta, along with the sauce mix and the reserved pasta water. Stir well and let the sauce coat the pasta for 1 minute. Ready to serve.
