For the fish stock: Coat a large, tall pot lightly with olive oil. Add the onions, celery, and fennel tops and bring the pot to a high heat. Season the veggies lightly with salt. Cook the veggies until they are soft and very aromatic, 7 to 8 minutes. Add the garlic and cook for 1 to 2 minutes more. Add the fish bones and shrimp shells, stir to combine, and cook for 3 to 4 minutes. Add the anise-flavored liqueur and let it reduce by half.
Add water to the pot until it covers the fish bones by 2 inches and season lightly with salt. Toss in the bay leaves and thyme bundle. Bring the pot to a boil, reduce to a simmer, and simmer for 30 minutes. Strain the liquid and reserve.
For the fish: Coat a straight-sided saute pan with olive oil. Add the onions and fennel, season with salt, and cook over medium heat until the veggies start to soften, 3 to 4 minutes. Add the garlic and cook for 1 to 2 minutes. Add the anise-flavored liqueur and white wine and reduce by half.
Add the reserved fish stock and season with salt. Bring to a boil, reduce to a simmer, and simmer the broth for 12 to 15 minutes. Taste to make sure it is delicious.
Meanwhile, coat a second saute pan with olive oil. Bring the pan to a high heat. Pat the cod dry with paper towels and season with salt. Add the cod to the screaming hot pan and cook the fish until it is golden and crisp on both sides and 3/4 of the way cook through. Remove the fish from the pan and reserve. Do the same with the shrimp and reserve. Add the potato rounds to the pan, season with salt, and brown on both sides. Remove the potatoes from the pan and reserve with the cod and shrimp.
When the fish broth is ready, add the cod, shrimp, potatoes, and zucchini. Turn off the heat and cover, letting the fish gently cook through, 2 to 3 minutes.
To serve, remove the cod and shrimp from the pan. Scoop the veggies out of the broth and divide them between 4 serving bowls. Lay a piece of cod on top of each of the veggies and arrange 2 shrimp next to the cod. Ladle the fish broth over each piece of cod. Drizzle with big fat finishing oil and top with fennel fronds.