Bring to a boil in a large pot, cover, and reduce heat to low until mussels open, shaking often (~5 minutes):
- 2 pounds mussels, scrubbed debearded
- 1 onion, chopped
- 2/3 cup dry white wine
- 8 parsley sprigs plus 1/3 cup chopped parsley
Strain the broth and reserve. Set the mussels aside. When convenient, remove most from their shells. Discard vegetables.
Wipe the pot dry.
Return the pot to medium heat and saute until softened:
- 2 tablespoons butter
- 1 leek, finely chopped
- 1 fennel bulb, finely chopped
Add, and simmer until liquid has reduced by a third (~25 minutes):
- Reserved broth
- 24 ounces clam juice
- 2 parsley sprigs
- 3 large thyme sprigs
- 2 bay leaves
Discard parsley sprigs, thyme sprigs, and bay leaves.
Add, and simmer until opaque in the center (~4 minutes):
- 12 ounces sea scallops
Reduce heat to low. Add:
- Cooked mussels
In a small bowl, whisk together, temper with hot broth, slowly add to pot, and allow to thicken:
- 2/3 cup crème fraîche
- 2 large egg yolks
Stir in:
- 1/3 cup chopped parsley.
Season:
- Kosher salt
- Freshly ground black pepper.
Serve in warmed shallow bowls.