Heat the olive oil and melt the butter in a large Dutch oven set over medium-high heat.
Add the chopped fennel and onion and cook for about 5 minutes, or until softened. Stir frequently to prevent burning.
Stir in the garlic, thyme, paprika, saffron, and cayenne pepper; cook for 20 seconds.
Stir in the wine, scraping up all the browned pieces from the bottom of the pot, and cook for 1 minute.
Add in the clam juice (or seafood stock), tomatoes with the tomato juices, and the bay leaves.
Bring to a boil; reduce heat to medium and simmer for 8 to 10 minutes, or until the liquid has reduced by half.
Season the halibut pieces with salt and pepper.
Add the halibut to the pot; reduce heat to medium-low, cover, and cook for 2 minutes.
Next, nestle in the mussels and scallops, cover the pot, and cook for 3 minutes.
Remove cover; add in the shrimp, cover, and cook for 2 to 3 more minutes, or until shrimp is opaque, halibut is cooked through, mussels have opened, and scallops are firm.
Remove from heat and discard bay leaves and any unopened mussels.
Stir in the basil and parsley.
Taste for salt and pepper and adjust accordingly.
Ladle into wide bowls, garnish with red pepper flakes, if using, and serve with baguette slices.