Set a large Dutch oven over medium-high heat, and heat 2 tablespoons of the olive oil until shimmering. Add the fennel and onions, and cook, until the vegetables are softened, about 3 minutes. Stir in the garlic and saffron, and cook until fragrant, about 1 minute. Deglaze with the white wine and licorice liqueur, scraping up any browned bits. Reduce the liquid until it's almost completely evaporated. Then add the tomatoes, fish stock, thyme and bay leaf. Add the potatoes, bring to a boil and cook until just tender, about 10 minutes. Add the clams and mussels and simmer for another 3 minutes to give them a head start. Add the shrimp, simmering until the shellfish open, a final 2 minutes. Discard any unopened shellfish.
Bring a large nonstick saute pan to medium-high heat and add the remaining oil. Pat the halibut fillets dry with a paper towel and sprinkle both sides generously with salt and pepper. Once the oil begins to smoke lightly, sear the halibut. Cook until the bottom is golden and crispy, 4 to 5 minutes on the first side. Flip and finish cooking for 1 to 2 minutes on the second side. Remove the fish from pan, keep warm and set aside.
To serve, place each halibut fillet in the bottom of a shallow serving bowl. Arrange a few potatoes, shrimp, clams and mussels around the fish in each bowl. Ladle broth into the bowls. Garnish with parsley, scallions, a few fennel fronds and lemon wedges. Serve immediately with Garlicky Rouille.
Peel the potatoes and rough chop. Place the potato, red pepper and garlic into the bowl of a food processor and pulse until just coarsely chopped. Add the mayonnaise, lemon juice and sweet paprika. Process until smooth and creamy. Season with cayenne and salt to taste. Cover and place in the refrigerator for 10 to 15 minutes to chill and allow the flavors to come together.