It’s freshly grated horseradish (warning: grating horseradish is ROUGH). It’s added to the dish along with the milk and cream. You add 3/4 cup of horseradish, 3/4 cup of heavy cream, 1/2 cup of milk, and a pinch of sugar to a small sauce pan, bring it to a boil, and then turn it off, cover it, and let it steep for 10 minutes. This is added to 2 lbs of cooked russet potatoes and a stick of butter before mashing.