Ingredients

• 4 pounds fish bones
• 4 medium onions, chopped
• 4 fennel bulbs, chopped
• 4 small carrots (about 8 ounces), chopped
• 1/2 cup olive oil, divided
• Kosher salt, freshly ground pepper
• freshly ground pepper
• 1 teaspoon saffron threads
• 2 leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
• 4 garlic cloves, thinly sliced
• 1 cup dry vermouth
• 8 fresh flat-leaf parsley stems
• 4 1" strips lemon zest removed with a vegetable peeler
• 2 bay leaves
• 2 teaspoons black peppercorns
• 1 garlic clove, chopped
• 1 cup fresh flat-leaf parsley leaves
• 1/2 cup fresh tarragon leaves
• 1/4 cup extra-virgin olive oil
• Kosher salt, freshly ground pepper
• freshly ground pepper
• 3 1/2 pounds clams or cockles, scrubbed
• 8 4-ounce firm fish fillets (such as halibut, snapper, or cod)
• Kosher salt, freshly ground pepper
• freshly ground pepper
• Olive oil (for drizzling)
• 8 slices country-style bread, toasted

Instructions

1. Preheat oven to 450°F. Divide bones, onions, fennel, carrots, and 1/4 cup oil between 2 rimmed baking sheets; season with salt and pepper and toss to coat. Roast, tossing halfway through, until vegetables are softened and browned, 60–75 minutes.
2. Heat saffron and remaining 1/4 cup oil in a large pot over medium-high heat, stirring, until fragrant, about 2 minutes. Add leeks and garlic; season with salt and pepper and cook, stirring often, until golden, 8–10 minutes. Add vermouth; bring to a boil, reduce heat, and simmer, stirring occasionally, until slightly reduced, about 3 minutes.
3. Add roasted bones and vegetables, parsley stems, lemon zest, bay leaves, peppercorns, and 10 cups water to pot. Bring to a boil, reduce heat, and simmer gently, skimming as needed, until reduced by half, about 1 hour. Strain through a sieve into a clean pot; discard solids. Season with salt.
4. For pesto: Pulse garlic, parsley, and tarragon in a food processor until very finely chopped. With motor running, slowly add oil, and process until smooth; season with salt and pepper.
5. Pulse garlic, parsley, and tarragon in a food processor until very finely chopped. With motor running, slowly add oil, and process until smooth; season with salt and pepper.
6. For assembly: Return broth to a simmer; add shellfish, cover pot, and cook until shellfish open (discard any that do not open), about 5 minutes. Transfer shellfish to a large bowl and add a ladleful of broth; tent with foil. Season fish with salt and pepper; add to broth in pot. Cover; cook fish gently until opaque throughout, about 4 minutes. Divide fish and shellfish among shallow bowls and ladle broth over. Drizzle stew with oil, top with pesto, and serve with toast. DO AHEAD: Brodo can be made 3 days ahead. Cover and chill. Pesto can be made 1 day ahead. Press plastic wrap directly on surface and chill.
7. Return broth to a simmer; add shellfish, cover pot, and cook until shellfish open (discard any that do not open), about 5 minutes. Transfer shellfish to a large bowl and add a ladleful of broth; tent with foil. Season fish with salt and pepper; add to broth in pot. Cover; cook fish gently until opaque throughout, about 4 minutes.
8. Divide fish and shellfish among shallow bowls and ladle broth over. Drizzle stew with oil, top with pesto, and serve with toast.
9. DO AHEAD: Brodo can be made 3 days ahead. Cover and chill. Pesto can be made 1 day ahead. Press plastic wrap directly on surface and chill.