Heat olive oil and melt butter in a large Dutch oven over medium-high heat.
Add fennel and onion; cook for about 5 minutes, stirring frequently until softened.
Stir in garlic, thyme, paprika, saffron, and cayenne; cook for 20 seconds to release aromas.
Pour in white wine, scraping the bottom to deglaze, and cook for 1 minute.
Add clam juice or seafood stock, diced tomatoes with juices, and bay leaves; stir to combine.
Bring to a boil, reduce heat to medium, and simmer 8 to 10 minutes until liquid reduces by half.
Season halibut with salt and pepper, gently add to pot, reduce heat to medium-low, cover and cook 2 minutes.
Add mussels and scallops into the broth, cover and cook 3 minutes.
Uncover, add shrimp, cover again and cook 2 to 3 minutes until shrimp are opaque and seafood is cooked through.
Remove from heat, discard bay leaves and any unopened mussels.
Stir in chopped basil and parsley, adjust seasoning with salt and pepper to taste.
Serve ladled into wide bowls, garnished with red pepper flakes if desired, alongside toasted baguette slices.