Bring a large pot of salted water to a boil. Add carrots, cover partially, and simmer 5 minutes. Add parsnips, and cover partially. Simmer until vegetables are very tender, about 15 minutes. Drain well.

Return vegetables to saucepan, and stir over medium heat until any excess moisture evaporates. Transfer to food processor. Add butter, and process until smooth. Season with nutmeg, salt, and pepper. Can be made 4 hours ahead. Warm over low heat, stirring often. Transfer to bowl. Serve.