Peel sweet potatoes and parsnips, and cut into ¼-inch thick slices.
Combine sweet potato and parsnip slices, cream, butter, salt, and half of the horseradish root in a 3 or 4 quart saucepan with a tight fitting lid. Heat over low heat, stirring occasionally, until vegetables have softened. This will take about 40 to 50 minutes.
Remove from heat and mash with a potato masher to desired texture. Stir in half of remaining horseradish making sure to incorporate well. Taste and adjust seasoning with salt, additional horseradish, and/or sweetener if needed. Serve hot.
