Peel and roughly chop 3 pounds celery root. Place in a pot and cover with equal parts milk and water (about 2 1/2 cups each); add a pinch of salt. Bring to a simmer and cook until soft, 20 to 25 minutes; drain. Puree with 1/3 cup heavy cream and 2 tablespoons butter. Season with salt, then add 2 tablespoons whole-grain mustard and pulse to combine.
TIP: Use a food processor or immersion blender to puree these veggies-they're too fibrous for a potato masher.