Seafood Stew with Fennel and Thyme

Combine mussels, 1 1/4 cups onions, wine, and 8 parsley sprigs in large pot; bring to boil. Cover and cook until mussels open, shaking pot often, about 5 minutes. Using slotted spoon, transfer mussels to large bowl to cool (discard any mussels that do not open). Strain cooking liquid into large measuring cup; discard vegetables in strainer. Add enough water to cooking liquid to measure 2 cups total. Remove mussels from shells if desired.
Melt butter in same large pot over medium heat. Add remaining 1 1/4 cups onions, leeks, and fennel and sauté until leeks are soft, stirring frequently, about 7 minutes. Add reserved mussel cooking liquid, 4 parsley sprigs, clam juice, thyme sprigs, and bay leaves. Simmer uncovered until vegetables are tender and liquid has reduced by 1/3, about 25 minutes. Add halibut and scallops to broth and simmer until just opaque in center, about 4 minutes. Using slotted spoon, transfer halibut and scallops to bowl. Discard parsley sprigs, thyme sprigs, and bay leaves.
Whisk crème fraîche and egg yolks in medium bowl to blend. Whisk in 1/2 cup hot cooking liquid from pot. Gradually stir yolk mixture into stew. Cook over medium heat until liquid thickens slightly, stirring constantly, about 5 minutes (do not allow mixture to boil). Return halibut, scallops, and mussels to pan. Cook until halibut is heated through, stirring often, about 5 minutes. Stir in 1/2 cup chopped parsley. Season with salt and pepper. Serve in warmed shallow bowls.
*Crème fraîche is sold at some supermarkets. If unavailable, heat 1 cup whipping cream to lukewarm (85°F). Remove from heat and mix in 2 tablespoons buttermilk. Cover and let stand in warm, draft-free area until slightly thickened, 24 to 48 hours, depending on temperature of room. Refrigerate until ready to use.