Slide the pasta into a large saucepan of boiling water and cook for 8-10 minutes, or until just tender.
Meanwhile, heat the oil in a non-stick frying pan, add the onion and fry gently for 2-3 minutes, or until softened.
Add the mushrooms, then continue to fry gently for a further 3 minutes.
Drain the pasta and return it to the pan.
Stir in the onion mixture, mackerel, creme fraiche, and herbs.
Season well, heat through gently, and turn into a warmed serving bowl.