Place the celery root and potatoes in a medium saucepan and cover with cold water; add a teaspoon of salt. Bring to boil over medium heat and simmer for 15 to 20 minutes, until the celery root and potatoes are very tender. Drain the water from the vegetables and pass them through a food mill or a ricer into a large mixing bowl. Stir in the cream, butter, and horseradish until the cream is absorbed and the mixture is smooth. Season with kosher salt and pepper and garnish with chopped chives. Drizzle with a healthy dose of olive oil and serve.