Put a large pan of water on to boil for the pasta. Fry the onion in the oil for 10 mins until softened and starting to colour. Pour in the wine, turn up the heat and let it bubble away until it reduces.
Boil the pasta for 10 mins. Add crème fraîche to the onions, bring to the boil, tip in the mussels and cook for 5 mins until open (discard any that don’t). Drain the pasta, return to the pan and tip in the mussel mixture. Toss with the herbs and serve.