Blend 3 large egg yolks, 1 large egg, and ½ cup fresh lemon juice in a blender until smooth.

Heat 3 Tbsp. extra-virgin olive oil in a medium Dutch oven or other heavy pot over medium. Cook 1 medium fennel bulb, quartered, cored, thinly sliced, 1 medium onion, thinly sliced, and 6 garlic cloves, finely chopped, stirring often, until fennel is softened and onion is translucent, 6–8 minutes. Add 1 lb. small clams (such as littleneck or Manila; about 12), scrubbed, 1 lb. large shrimp, peeled, deveined, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt. Cook, stirring occasionally, until shrimp is just starting to turn opaque, about 4 minutes. Stir in 1½ cups water. Bring to a boil, then reduce heat to a simmer.

Measure out ¾ cup broth in a heatproof measuring glass. With blender on low speed, slowly pour broth into egg mixture and blend to combine. Whisking constantly, slowly pour egg mixture into pot. Arrange 1 lb. flaky white fish fillets (such as cod, halibut, or snapper), cut into 4" pieces, in pot, overlapping if needed, and bring to a simmer. Reduce heat to low and add ¼ cup heavy cream and 1 tsp. freshly ground pepper. Cook, stirring occasionally, until fish is opaque throughout and clams have opened, about 4 minutes; discard any clams that haven’t opened. Remove from heat and stir in 2 Tbsp. finely chopped parsley and/or dill. Taste and add more salt if needed.

Ladle stew into shallow bowls; top with fennel fronds and parsley leaves and/or dill sprigs. Serve with lemon wedges.