Place the live mussels in fresh water. Scrub and debeard the mussels, discarding any that are open or cracked, or remain open when they’re tapped. (Fresh mussels should look black and shiny and should only smell pleasantly of the deep sea – they should all close when they are tapped. Discard any that smell 'fishy', look dry or remain open.)
Place the onion, shallot and garlic in a large saucepan with the oil, and gently cook over a low to medium heat for about 5 minutes, or until slightly softened.
Add the herbs, squeeze of lemon juice and Pernod or pastis, if using, and mix with the onion mixture.
Add the prepared mussels to the pan, pour over the wine and bring slowly to the boil, then simmer gently whilst stirring. Continue until the mussels have opened – this can take between 2 to 5 minutes. Discard any that do not open.
Add the creme fraiche (if using), and mix through the mussels. Cover until the frites are cooked.
Serve the moules in a deep bowl with the frites on the side, together with plenty of napkins and a large bowl for the discarded shells.