Heat the olive oil and melt the butter in a large Dutch oven over medium-high heat. Add the chopped fennel and onion, cooking for about 5 minutes until softened while stirring frequently. Stir in the garlic, thyme, paprika, saffron, and cayenne pepper and cook for an additional 20 seconds.

Pour in the white wine, scraping up any browned bits from the bottom of the pot, and cook for 1 minute. Add the clam juice or seafood stock, canned diced tomatoes with their juices, and the bay leaves, stirring to combine.

Bring the mixture to a boil, then reduce heat to medium and let it simmer uncovered for 8 to 10 minutes or until the liquid reduces by about half.

Season the halibut pieces with salt and pepper. Add them to the pot, reduce heat to medium-low, cover, and cook for 2 minutes.

Nestle the cleaned mussels and scallops into the pot, cover again, and cook for 3 minutes. Then remove the cover, add the shrimp, cover once more, and cook for 2 to 3 minutes until the shrimp turn opaque.

Remove the pot from heat, discard the bay leaves and any unopened mussels. Stir in the fresh chopped basil and parsley. Adjust seasoning with salt and pepper as needed.

Ladle the bouillabaisse into wide bowls, garnish with optional red pepper flakes, and serve alongside toasted baguette slices.