Melt butter in heavy saucepan. Add fennel and onion, cook over medium heat until soft but not brown, about 5 minutes. Add potatoes, stock and parsley, bay leaf, peppercorns and thyme enclosed in a small cheesecloth pouch. Simmer 30 minutes then remove. Pass soup through food mill or puree until smooth in a food processor or blender. Pour puree into a clean saucepan, bring back to boil and correct seasoning.
Add cream to taste and chopped parsley. Place bunch of watercress in boiling water for 3 to 5 seconds then plunge in cold water. Drain cut leaves from stalks. Place soup in a heated tureen and serve.
Makes four 1 1/2 cup servings.