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1
In the largest available pot, melt the butter over a moderate flame. Add the shallots and sweat, stirring occasionally, until fully translucent and soft but without colour.
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2
Verify each mussel: tap any that are open on the counter surface and discard those that do not close. The pot must be no more than two-thirds full of mussels to allow steam to circulate freely.
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3
Add the mussels, cider, bouquet garni and a generous measure of black pepper to the pot. Cover tightly and raise the heat to its maximum. Cook for 4 to 5 minutes, shaking the pot vigorously every 60 seconds to redistribute the mussels. This is where the dish lives or dies: the violent steam must open every shell and cook the flesh simultaneously — any longer and the mussels shrink and toughen within seconds of opening.
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4
Remove the mussels with a spider or slotted spoon the moment every shell stands open; discard any that remain closed. The flesh should appear plump and just set.
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5
Allow the cooking liquor remaining in the pot to settle undisturbed for 30 seconds so that any grit sinks to the bottom. Carefully decant the liquor off the sediment into a separate pan, leaving the last few millilitres behind.
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6
Bring the decanted liquor to a brisk boil and reduce by one third. Add the crème fraîche, return briefly to the boil to amalgamate, then taste: the mussels' natural brine may render additional salt unnecessary. The sauce should be lightly bound, glossy, and carry both the acidity of the cider and the richness of the cream in balance.
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7
Return the mussels to the pot, pour the sauce over them, scatter with chopped flat-leaf parsley and serve immediately in the pot. The flesh should be plump and yielding; the liquor fragrant with cream and cider.