Halve leek lengthwise and cut into ¼-inch-thick slices; transfer to a large bowl and cover with cold water. Shimmy leek slices around in water; let rest 10 minutes. Using a slotted spoon or spider (or hands) strain leeks from water, being careful not to kick up dirt at bottom of bowl.
Heat butter in a large pot or Dutch oven over medium heat.
Add leek, fennel, onion, and celery; season with ½ teaspoon each salt and pepper and cook, stirring frequently, until softened, 8–10 minutes.
Stir in garlic, oregano, thyme, and red pepper flakes and cook 1 minute.
Deglaze with wine, scraping up any browned bits. Stir in broth, 4 cups water, clam juice and bay leaves. Bring to a simmer and cook 10 minutes.
Season white fish with salt and pepper.
Stir in shrimp and mussels, then arrange fish and scallops on top of stew, cover, reduce heat to medium-low (if your stove runs hot, reduce the heat to low), and simmer until mussels open and fish, scallops and shrimp are firm and opaque, about 5 minutes.
Discard bay leaves and stir in parsley, 2 tablespoons chopped fennel fronds, and lemon juice; season with salt to taste (at least 1 teaspoon, I add 1 ½ teaspoons)
Off heat, stir in a tablespoon of butter or drizzle servings with olive oil (optional).