Heat olive oil in a large skillet over medium-high heat. When hot, add the shrimp shells and cook, stirring occasionally. When they turn red, add clam juice and water. Simmer, then strain and reserve the stock.
In a large Dutch oven or deep pot, heat remaining olive oil and unsalted butter. Add diced onion, diced fennel, and diced shallot. Season with Kosher salt, then cook. Add chopped garlic, fennel seeds, and crushed red pepper flakes, then stir and cook. Add tomato paste.
Pour in white wine, then scrape the bottom of the pan. Allow the wine to simmer and reduce by half.
Add shrimp stock, then pour in crushed tomatoes. Add bay leaves, tarragon (or oregano), stir well, then simmer.
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. Remove and discard bay leaves. Taste the broth.
Top each with a drizzle of high quality extra virgin olive oil, some chopped parsley, and reserved fennel fronds. Serve immediately with toasted sourdough!