1 Tablespoon white wine vinegar
Garnish: 1 and 1/2 Tablespoons parsley, chopped
Put the stock, celery root, celery, onion, and garlic in a medium saucepan, bring to a boil, reduce heat, cover, and simmer for about an hour. Remove from heat, then puree the solids in a food processor or blender--then mix in the stock til smooth. Return to the saucepan with the cream and salt, then heat through. When you're ready to serve it, stir in the horseradish, vinegar, and pepper. Ladle it into bowls and garnish with parsley.
If you're serving it as a meal in itself, serve with crusty black bread, strong cheese, and a salad of greens.