Yield: 6 servings.
3 tablespoons unsalted butter
1 small onion, chopped
3 stalks celery, chopped
2 cloves garlic, chopped
2 green onions, sliced
3 tablespoons all-purpose flour
1 (8-ounce) bottle clam juice
3 cups seafood stock
1 cup water
3 potatoes, peeled and cut into 1-inch cubes
1 bay leaf
½ teaspoon dried thyme
¼ teaspoon ground cayenne pepper
1 pound cod, catfish, or halibut fillets, cut into chunks
1 cup whole milk
Salt and ground black pepper to taste
Hot sauce (optional)
Celery leaves (for garnish)
Melt butter in a 5-quart stockpot or Dutch oven over medium heat. Add onion, celery, and garlic, and cook, stirring, until onion is tender, 5 to 7 minutes. Add green onions and stir.
Stir flour into the vegetable mixture until vegetables are evenly coated. Pour clam juice, stock, and water into the saucepan; whisk until smooth. Add potatoes, bay leaf, thyme, and cayenne pepper to the mixture. Bring the liquid to a boil, reduce heat to medium-low, and cook at a simmer until potatoes are tender, about 15 minutes.
Place fish into the soup. Cover soup pot and cook until fish is flaky, about 8 minutes, depending on type of fish used.
Pour milk into the soup and stir. Simmer soup until hot, 1 to 2 minutes. Season with salt and black pepper. Remove bay leaf to serve. Add hot sauce, if desired, and garnish with celery leaves.