Heat butter and olive oil in a heavy large pot or Dutch oven over medium-high heat. Add chopped onion, garlic, fennel and carrots and sauté for about 6 minutes, until tender.Add half the parsley, bay leaf, thyme, red pepper flakes, 1 teaspoon salt, pepper and turmeric. Gently cook for 2 minutes or until fragrant. Add tomatoes and stir to combine.Add potatoes, stock and white wine. Bring to a boil, lower to a simmer and cook, covered until potatoes are tender, about 10 minutes.Add fish and cook another 5 minutes uncovered or until fish is cooked through. (If you decide to use mussels or small clams, simmer with the pot covered until they open.) Add more salt to taste, if desired. Ladle into bowls and serve. Garnish with reserved parsley. Nice with a piece of toasted baguette rubbed with garlic.