Generously season the chicken breasts with salt & pepper. Add a splash of oil to a large frypan and cook over a medium heat for 7 minutes each side, or until cooked through. Remove from pan and leave to rest for a few minutes before slicing into thin pieces against the grain.
Cook pasta as per packet instructions and reserve ½ cup water when straining.
Add the onion, garlic & half the lemon zest to the pan and gently fry over a low heat until very soft and translucent.
Add the cream, parmesan & pasta water and bring to a simmer for 2 minutes, stirring constantly until smooth and thickened. Stir through the lemon juice, taste and season with salt & pepper.
Add the fettuccine and chicken to the pan with the cream sauce and toss until coated.
Before serving, sprinkle over extra parmesan, remaining lemon zest, chives and lots of cracked black pepper. Serve immediately.