The delectably creamy mash can be prepared through step 3 up to 4 hours in advance; cover with foil and keep at room temperature. Just before serving, proceed with recipe.
Place potatoes in a medium saucepan, and add enough cold water to cover by about 2 inches. Bring to a boil over high heat; add salt generously. Reduce heat, and simmer until tender when pierced with a paring knife, 20 to 25 minutes. Transfer to a colander to drain.
Repeat process with parsnips, cooking about 15 minutes. Drain; let cool slightly. Puree in a food processor until smooth.
Pass potatoes through a ricer or food mill into a large metal bowl or top of a double boiler. Stir in parsnips. Cover with foil to keep warm, up to 4 hours.
When ready to serve, reheat potato mixture over a pan of simmering water. Bring cream and butter to a simmer in a small saucepan. Stir into potato mixture until smooth. Stir in nutmeg; season with salt and pepper. Transfer to a large serving bowl.