Shuck oysters, reserving oyster liquor. Serve with Horseradish Toast.
Melt butter in a large Dutch oven over medium heat. Add celery root, parsnips, and 1/8 teaspoon salt; sauté 9 minutes or until soft. Add sherry, and increase heat to medium-high. Cook 8 minutes or until sherry is syrupy. Add cream, reduce heat to medium, and cook 23 minutes or until thickened. Add oysters and liquor, 1/4 teaspoon salt, and pepper; cook 2 minutes or until edges of oysters begin to curl.