food-safe cedar plank soaked in water at least 30 minutes
In a large skillet over medium-high heat, add clams and 1/2 cup white wine and cook until clams open, about 5 to 6 minutes (discard any clams that don’t open). Remove clams and any liquid in the skillet and set aside. In the same skillet over medium-high heat, add mussels and 1/2 cup white wine and cook until mussels open, about 5 to 6 minutes (discard any mussels that don’t open). Set aside.
Preheat the oven to 400 degrees F. Place halibut skin-side down on a cedar plank that has been soaked in water for at least 30 minutes (alternately use a baking sheet). Season halibut with Espelette pepper, fennel pollen and salt. Roast until cooked through and flaky, 10 to 12 minutes.
Meanwhile, in a large Dutch oven over medium heat, heat the oil. When shimmering, add onions and fennel bulb. Stir and cook until the onion is translucent, 3 to 4 minutes. Add garlic, tomatoes and remaining 1 cup white wine. Cover and simmer 15 minutes. Uncover, add shrimp and let steam from the heat of the liquid, 2 to 3 minutes.
To serve, add mussels and clams and their broth to the tomato mixture. Place cioppino in a large serving bowl. Nestle halibut on top. Garnish with fennel fronds. Serve.