1 Sauté onions in oil and butter, add wine: Heat oil and butter in the bottom of a large pot (6-qt) on medium heat. Add the onions and cook until softened, about 5 minutes.
Add the wine, if using, and turn up the heat, cook, uncovered until the wine reduces by half. (If not using wine, add 1/4 cup of water with the clam juice.)
2 Add potatoes, clam juice, spices, then simmer: Add the potatoes, clam juice, bay leaf, thyme, salt and pepper, and Old Bay spice. (The potatoes should be just barely covered with the liquid in the pot. If not, add water so that they are.)
Bring to a simmer, then lower the heat to medium and cook, covered, until the potatoes are almost done, about 10-15 minutes.
3 Heat cream: In a separate pot, heat the cream until steamy (not boiling).
4 Add fish to soup, add hot cream: Add the fish to the pot of potatoes and add the heated cream. Return to the stove. Cook on low heat, uncovered, until the fish is just cooked through, about 10 minutes.
Keep your eye on the heat! If you are using straight heavy cream you should be more easily able to avoid curdling, even if the soup starts to boil. But if you are substituting light cream, half and half, or milk, the mixture will likely curdle if it gets near boiling point (one of the reasons I like using straight heavy cream).
Keep the temperature so that it barely gets steamy, but not simmering.
When the fish is just cooked through, remove from heat.
Mix in the parsley. The flavors will improve if the soup rests 30 minutes before serving.
Serve with crusty bread or oyster crackers (not for gluten-free version).