In a large saucepan with a lid, over medium heat, melt half the butter
Add shallots, leek, garlic, and bay leaves
Stir occasionally until vegetables are soft but not brown (about 6 minutes)
Season with salt and freshly cracked pepper
Crank up heat to high and add white wine
When the wine has reduced by about half, add mussels, give it a good stir, and put the lid on
Shake the whole pan with a lid on, several times per minute. After a few minutes, check if the mussels have opened. It will only take three to five minutes.
As soon as they’re open, transfer to a warm bowl and cover with a lid or foil to keep mussels warm
Remove pan from heat, add remaining butter and crème fraîche, and whisk until it has emulsified. Taste the sauce you just created and season to your liking.
Put the mussels back into the pot
Add lemon zest, lemon juice, and parsley, and stir
Serve immediately on warm plates with toasted baguette. Don’t forget to spoon over all the remaining sauce in the pot!