In 2-quart heavy saucepan, cook whipping cream, leeks, thyme, salt, pepper and nutmeg over low heat, stirring occasionally, until mixture begins to simmer.
Stir in sweet potatoes and parsnips.
Cover; simmer about 10 minutes or until vegetables are slightly tender.
Layer half of vegetable mixture and half of cheese in casserole.
Top with apple.
Repeat layers of vegetables and cheese.
Bake uncovered about 45 minutes or until golden brown and bubbly.
Let stand 10 minutes before serving.
Notes / Tips / Wine Advice:
Splurging now and again is what this rich gratin is all about. For a less rich entrée, use equal parts whipping cream and half-and-half instead of all whipping cream. Along with the gratin, serve a lightly dressed salad.