Seafood en Brodo with Tarragon Pesto

Preheat oven to 450F. 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, 6075 minutes.

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, 810 minutes.

Add vermouth; bring to a boil, reduce heat, and simmer, stirring occasionally, until slightly reduced, about 3 minutes.

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.

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.

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.