How to Make It
In a medium saucepan, combine the sugar with 1/2 cup of water and bring to a boil over high heat. Boil for 5 minutes, then add the cranberries and lower the heat to moderate. Simmer without stirring until the cranberries pop and the syrup reduces slightly, about 5 minutes. Set aside.
In a large pot or dutch oven, combine the celery root, potatoes, coarse salt and garlic. Cover generously with cold water, bring to a boil over high heat and cook until tender, about 20 minutes. Drain well. Transfer the vegetables to a food mill or a ricer and puree, adding the milk, butter, table salt and pepper as you go along. Alternatively, mash with a potato masher.
To serve, reheat the cranberries in their syrup. Mound the puree on a platter or individual plates and make a well in the center. Spoon some of the cranberries and their syrup into the well. Serve the remaining cranberries and syrup alongside.