Bring the 1/4 cup dry white wine to a simmer in a small saucepan. Add the generous pinch saffron threads. Remove from the heat, cover, and let the pan stand for 15 minutes.
Heat 1 tablespoon of the olive oil in a heavy, large saucepan over medium-high heat. Add the 1 1/4 cups coarsely chopped leeks and 1 cup coarsely chopped onion and sauté until tender, about 8 minutes.
Add the 1 tablespoon minced garlicand sauté for 1 minute. Add the 3 cups fish stock, one (14.5-oz) can diced tomatoes with their juice, 1/2 teaspoon dried oregano, and the saffron-wine mixture. Bring to a simmer and cook for 20 minutes to allow the flavors to meld.
Add the 6 ounces linguiça, 20 mussels, and 12 littleneck clams. Cover and increase the heat to high. Boil until the shellfish open and spill their liquor, about 5 to 8 minutes. Discard any that do not open.
Meanwhile, pat the four (5-ounce) cod fillets dry. Sprinkle generously with salt and freshly ground pepper. Place the yellow cornmeal on a plate and dip one side of each fillet in the cornmeal to coat.
Heat the remaining 2 tablespoons of olive oil in a large nonstick skillet over high heat. Add the fish, cornmeal side down, and cook until the bottom is golden brown, about 4 minutes. Turn the fish over. Ladle 1 cup of the broth from the stew over the fish. Cover the skillet and simmer until the fish is just cooked through, about 4 minutes more.
To serve, transfer a fish fillet, cornmeal side up, to each of 4 shallow bowls. Arrange the clams, mussels, and sausage around the fish. Carefully ladle the remaining broth and vegetables over. Sprinkle with 1/4 cup chopped flat-leaf parsley and rush to the table.