Combine 1 1/2 cups onion, wine, mussels, and 3 parsley sprigs in a Dutch oven; bring to a boil over medium-high heat. Cover and cook 2 minutes or until mussels open; discard any unopened shells. Remove mussels with a slotted spoon; set aside. Strain cooking liquid through a sieve into a bowl, reserving liquid, and discard solids. Remove meat from mussels. Discard shells.
Place Dutch oven over medium heat; melt butter. Add 1 1/2 cups onion, fennel, and leeks; cook 10 minutes or until tender, stirring occasionally. Combine water and bouillon, stirring until bouillon cubes dissolve. Add bouillon mixture and reserved cooking liquid to onion mixture, stirring to combine.
Place the remaining 3 parsley sprigs, thyme, and bay leaf on a double layer of cheesecloth. Gather the edges of the cheesecloth together, and tie securely. Combine the cheesecloth bag and broth mixture; bring to a boil. Reduce heat, and simmer 10 minutes. Remove bag, and discard.
Combine the half-and-half, crème fraîche, and egg yolks in a medium bowl, stirring well with a whisk. Gradually add 1 1/2 cups hot broth mixture to egg mixture, stirring constantly with a whisk. Return egg mixture to pan. Cook 5 minutes or until soup thickens slightly (do not boil), stirring constantly. Add fish and scallops; cook 5 minutes or until fish is done, stirring frequently. Stir in mussels, 1/4 cup chopped parsley, and lemon rind; cook 1 minute or until thoroughly heated. Stir in salt and pepper.