Heat a heavy based pan over a medium-high heat and spritz with low calorie cooking spray. Add the chicken and stir fry for 5 minutes, or until sealed.
Add the onion and red chilli (if using) to the pan, and continue to fry for 2 minutes.
Add the red pepper, and fry for a further 5 minutes, or until the chicken is cooked through and the vegetables have softened but still retain their crunch. Add the Schwartz Fajita Seasoning for the last minute.
Meanwhile, cook the pasta according to packet instructions. Drain and set to one side.
Add the chicken stock and quark to a jug, and mix thoroughly until a sauce is formed. Continue to simmer for 10 minutes, or until a thick and creamy sauce is formed.
Add the pasta to the pan and stir well, ensuring that everything is piping hot.
Serve and enjoy!