1 1/2 pounds live mussels, scrubbed, debearded
1 cup dry white wine, such as Sauvignon Blanc
2 medium onions, chopped, divided
5 fresh Italian parsley stems
2 tablespoons (1/4 stick) unsalted butter
2 large leeks (white part only), chopped
1 fennel bulb, trimmed, chopped
4 cups fish stock
1 bouquet garni (see note)
1 3/4 pounds white fish fillets, such as halibut or flounder, patted dry, cut into 2-inch pieces
8 ounces sea scallops
1 cup creme fraiche or heavy cream
2 large egg yolks
1/2 cup fresh Italian parsley leaves, coarsely chopped
Peel from 1/2 lemon, cut into matchstick-size strips (white pith removed)
Sea salt and freshly ground black pepper
1. Combine mussels, wine, 1 chopped onion and parsley stems in copper or stainless steel casserole. Cover and bring to boil over medium-high heat. Cook just until mussels open, shaking pan often, about 8 minutes (discard any mussels that do not open). Strain and reserve cooking liquid, adding water if necessary to make 2 cups total. Transfer mussels to large bowl; remove meat from shell. Reserve meat; discard shells.
2. Melt butter in same casserole over medium heat. Add remaining chopped onion, leeks and fennel. Saute until onion and fennel are translucent and leeks are almost tender, about 5 minutes. Add reserved cooking liquid, fish stock and bouquet garni. Simmer until vegetables are tender and liquid has reduced by about 1/3, about 20 minutes. Add fish and scallops. Cook until fish and scallops are almost cooked through, about 4 minutes. Transfer fish and scallops to warm bowl. Discard bouquet garni.
3. Whisk creme fraiche and egg yolks in medium bowl to blend. Gradually whisk in 1/2 cup seafood cooking liquid. Whisk cream mixture back into seafood cooking liquid in casserole. Cook until liquid thickens just slightly, stirring constantly, about 3 minutes (do not boil). Add mussels, fish, scallops and any accumulated juices from bowl to casserole. Cook just until seafood is heated through. Season stew to taste with salt and pepper. Stir in parsley and lemon peel. Cook 2 minutes longer. Ladle stew into bowls and serve immediately.