Make the shrimp stock. Heat 2 Tbsp olive oil in a large skillet over medium-high heat. When hot, add the shrimp shells and cook, stirring occasionally, for 1-2 minutes. When they turn red, add 8oz of clam juice and 2 cups of water. Simmer for 10-15 minutes, then strain and reserve the stock. (You can add carrots, celery, garlic, herbs to this while simmering, if wanted!)
Sauté the aromatics. In a large Dutch oven or deep pot, heat remaining 2 Tbsp olive oil and 2 Tbsp unsalted butter. Add 1 cup diced onion, 1 cup diced fennel, and 2 Tbsp diced shallot. Season with ½ tsp Kosher salt, then cook, stirring occasionally, for 6 minutes. Add 4 cloves chopped garlic, 1 tsp fennel seeds, and ½ tsp crushed red pepper flakes, then stir and cook for 1 minute more. Add 2 Tbsp tomato paste, then use a spatula to break it up until dissolved.
Deglaze with wine. Pour in 1 cup of white wine, then scrape the bottom of the pan. Allow the wine to simmer and reduce by half, about 2 minutes.
Make the sauce. Add shrimp stock (or 3 cups of seafood stock), then pour in 1 (28oz) can of crushed tomatoes. Add 2 bay leaves, 2-3 sprigs of tarragon (or oregano), stir well, then simmer for 10 minutes.
Add seafood. Pat the halibut and shrimp dry with paper towels, then season with a little Kosher salt. Add the seasoned halibut, shrimp, mussels, and clams, then cover with a lid and cook for 5 minutes, or until the shells open up. Discard any mussels and clams that remain closed. Remove and discard bay leaves. Taste the broth, adjust seasoning as needed, then transfer to serving bowls.
Garnish, then serve. Top each with a drizzle of high quality extra virgin olive oil, some chopped parsley, and reserved fennel fronds. Serve immediately with toasted sourdough!