Add fennel and garlic; season with salt and pepper and cook, stirring occasionally, until fennel is soft, about 2 minutes. Add wine, bring to a boil, and cook until almost completely evaporated, about 4 minutes. Add 2 cups water to pot and bring to a boil. Reduce heat and simmer until potatoes are tender, 10–12 minutes. Stir in crème fraîche, zest, and Pernod. Add flounder, cover pot, and reduce heat. Simmer until fish is cooked through, about 4 minutes.