These are like mashed potatoes, but celery root adds an earthy flavor. And just like mashed potatoes, this recipe invites variations. Throw in fresh herbs, such as minced chives or rosemary, use a good olive oil in place of the butter, or substitute buttermilk or sour cream for milk. You can also add a handful of green peas for color. I happen to like potatoes lumpy, but if you like them very smooth, use a ricer or food mill. Prep: 10 minutes, Cook: 40 minutes.