Melt butter in a large deep pot over medium high heat. Add carrots, leek, celery and parsnip and stir until they begin to soften, about 5 minutes. Add potatoes and stir for another couple of minutes.Add broth and bring to a simmer. Cook until veggies are tender, about 30 minutes.Transfer soup to blender and puree until smooth - you might have to do this in batches. Season to taste with salt and pepper.If you want a garnish, mix up the sour cream and the horseradish and drizzle over the top. Grind on some pepper and serve in warmed bowls.