Cut the sweet potatoes and then the parsnips into paper-thin slices using a food processor fitted with the thinnest vegetable-slicing blade. If necessary, cut the potatoes in half or quarters lengthwise so they fit into the feeder tube.
Use 1 tablespoon of the butter to coat the inside of a 2-quart oven-proof casserole (a 6 by 10-inch
Pyrex pan works well). Combine the cream, stock, nutmeg, cinnamon, salt and pepper. Place
1/3 of the sweet potatoes in a thin layer across the bottom of the casserole. Layer half the parsnips on top of the sweet potatoes. Repeat this sequence until all of the sweet potatoes and parsnips have been used. Pour the cream mixture over the top. It should reach about 3/4 of the way up the vegetables. Butter a piece of aluminum foil with the remaining butter and wrap the foil tightly over the casserole. Put in the oven and turn tempertature down to 325 degrees.
Bake the casserole 1 hour. Remove the foil and continue to cook another 15 minutes, until the vegetables are tender when pierced with a knife and the top is browned. Allow to cool slightly before serving. Serve warm.