Bring a large wide Dutch oven to medium heat and add olive oil and butter.
Once melted, add the sliced leeks and fennel and sauté until softened and fragrant, but not browned. about 2-3 minutes.
Add the chopped garlic and sauté for another minute. Stir the tomato paste into the garlic for another 30 seconds.
Add the chopped tomatoes, with their juices along with the saffron, and give everything a good sauté for another 3-4 minutes until the tomatoes begin to soften and release their juices.
Pour the seafood stock in and nestle in the fresh thyme, bay leaf and orange peel. Simmer for 15-20 minutes.
While simmering, add the fish that take longer to cook, the halibut and simmer for 3-4 minutes.
Next, add the mussels and clams and and continue cooking for another 3-4 minutes.
Then add scallops and continue cooking for another 2-3 minutes and finally, add the shrimp and continue cooking for another 2-3 minutes until the shrimp are just cooked through. At this point, the fish should be cooked through and mussels and clams just opened.
Remove the bouillabaisse from heat and taste broth for seasoning.
Ladle the bouillabaisse into bowls with assorted fish and garnish with chopped parsley