Soak finnan haddie in a bowl of cold water for 30 minutes; drain. Melt butter in a 6-qt. saucepan over medium-high. Cook onion until soft, 3–4 minutes. Sprinkle in flour and cook for 2 minutes. Whisk in clam juice, bay leaf, and thyme and bring to a boil. Reduce heat to medium and cook until thickened, 18–20 minutes. Add potatoes and cream and simmer until potatoes are tender, about 15 minutes.