Clean the mussels by placing them in a large bowl of cold water
with a spoonful of flour. As they soak, examine them. Discard any
mussels that are cracked or that won’t close. After 15 minutes,
drain the water and rinse one more time.
On medium-high heat, melt the butter in a wide pot or Dutch
oven large enough to hold all the mussels. When the butter’s hot
and foaming, add the shallots and cook for 30 seconds to 1 minute,
until the shallots start to brown.
Add a little bit of the cider and all of the mustard. Stir until
combined, and then stir in the rest of the cider and the thyme.
Bring to a boil and taste for flavor, adjusting with salt and pepper.
Add the mussels and cover. They cook very fast, so begin
checking them after 1 minute. Remove to a platter any mussels
that open*. Re-cover and continue checking until all the mussels
are opened and removed. Discard any that don’t open.
To the liquid in the pan, add the cream and cook on high heat
until the sauce is thickened. Taste to adjust.
You can serve this two ways: you can pour all of the sauce over
the mussels on the platter, or you can remove the mussels from
their shells and place them in the sauce and serve it as more of a
soup. Either way, make sure to have bread handy to sop up that
sauce: it’s the best part.
* This is a wonderful technique that you should apply to all
mussel dishes: remove them as soon as they open and they’ll
never taste rubbery or overcooked.