Heat oil in a large heavy-bottom Dutch oven or soup pot over medium-high heat. Add onion, celery, garlic, thyme, fennel, salt, pepper, nutmeg and cook, stirring often until the onion is translucent and starting to brown, 6 to 8 minutes.
Add wine, and increase heat to high. Cook, stirring often until the liquid has almost completely evaporated, 3 to 4 minutes. Add clam juice, 2 cups chicken broth and potatoes, cover and bring to a simmer, stirring occasionally. Remove lid. Reduce heat to medium-low to maintain a simmer, and cook, stirring occasionally until the potatoes are tender, 8 to 10 minutes.
Whisk the remaining ½ cup broth with flour (or cornstarch) until smooth. Stir into the simmering soup, and continue stirring until the soup is thickened. Stir in clams and any juice from their container and cod or halibut. Let the mixture slowly come to a simmer, gently stirring occasionally and let cook until the cod is opaque all the way through, 4 to 5 minutes total. Stir in half and half and remove from the heat. Serve garnished with parsley or chives.