Step 1Place celery roots, potato, garlic and 1 teaspoon salt in large pot. Cover with water and bring to boil over high heat. Reduce heat to simmer and cook, covered, until vegetables just start to get tender, about 10 minutes. Add squash. Cook, covered, until all vegetables are very soft, about 25 minutes more.
Step 2Drain vegetables in colander; discard liquid. Transfer vegetables to food processor fitted with metal blade and add remaining salt, butter, nutmeg and pepper to taste. Puree until smooth, about 1 to 2 minutes. Taste; adjust seasoning. (Can be made a few days ahead and refrigerated, covered airtight. To serve, reheat in microwave oven until hot. Stir, taste and adjust seasonings.)