1In a heavy soup pot, melt butter over medium-high heat. Add onion; sauté 3 minutes.
3Sprinkle flour over mixture; stir until well blended. Slowly add broth; stir until flour dissolves. Bring to a boil; reduce heat to medium.
5Add celery, celery root, allspice, and mace. Simmer 40 minutes.
7Stir in half-and-half. Add fish and oysters; continue to cook 20 minutes.