Brush the mussels under cold water. (NB, opened mussels will close up again when rinsed). Discard open mussels, these are inedible. Chop the onion. Heat the oil in a wide-bottomed pan and sauté the onions.
Pour in the beer and stock, bring to the boil. Add the mussels, cover and sauté over a high heat, shaking the pan until they open. Discard unopened ones. Remove the mussels with a slotted spoon.
Reduce the stock to half the amount. Add the mustard and cream, bring to the boil, season. Chop the herbs, add to the pan, return the mussels to the sauce, gently heat, serve immediately.