Select, scrub and clean the mussels, discarding any that are cracked, damaged or refuse to close when tapped. scrub them several times and tug out any black fibrous beards.
Slice the bacon into strips. finely chop the shallots/ onion. Crush the garlic. finely slice the tarragon or strip the thyme.
Heat a wide saute pan/casserole dish large enough to take all the mussels. Add the oil. Throw in the bacon and fry gently for 5 minutes. Remove with a slotted spoon.
Add the shallots and garlic. Stir and fry very gently in the bacon oil for 5 minutes, or until soft and translucent. Increase the heat.
Add the cider and lemon juice/real ale and bring the mix to the boil. At that moment, whack the mussels in. Cover the pan immediately and cook for 3-4 minutes, shaking the pan a few times to keep it even. Turn the mussels once and check progress.
Once they're opened, drain the pot into a colander over a bowl. Keep the mussels warm. Return the collected liquid to the pan.
Add the tarragon and thyme and boil to reduce it a little if there's too much or if you want to reduce it to strengthen the flavour. Otherwise, just add the cream and mustard, bring to the boil, stir to amalgamate and taste for seasoning. Add the bacon.
Divide the mussels between bowls, scatter over the parsley and pour over the sauce. Serve with lots of bread to soak up the juices and frites.