1. Heat vegetable oil in Dutch oven over medium-high heat until shimmering. Add onions, 1/2 teaspoon salt, and 1/2 teaspoon pepper; cook, stirring frequently, until onions begin to brown, 7 to 9 minutes. Add water and cook, stirring frequently, until onions are soft, 2 to 4 minutes. Stir in garlic, bay leaves, oregano, and pepper flakes and cook for 1 minute. Stir in tomatoes and reserved juice and clam juice and bring to simmer. Reduce heat to low, cover, and simmer for 5 minutes.

2. Submerge halibut in broth, cover, and gently simmer until fish is cooked through, 12 to 15 minutes. Remove pot from heat and, using slotted spoon, transfer halibut to plate, cover with aluminum foil, and set aside.

3. Bring clams, wine, and butter to boil in covered 12-inch skillet over high heat. Steam until clams just open, 5 to 8 minutes, transferring them to pot with tomato broth as they open.

4. Once all clams have been transferred to pot, add mussels to skillet, cover, and cook over high heat until mussels have opened, 2 to 4 minutes, transferring them to pot with tomato broth as they open. Pour cooking liquid from skillet into pot, being careful not to pour any grit from skillet into pot. Return broth to simmer.

5. Stir parsley into broth and season with salt and pepper to taste. Divide halibut among serving bowls. Ladle broth over halibut, making sure each portion contains both clams and mussels. Drizzle with olive oil and serve immediately.

Serves 4 to 6