Place the chopped parsnips and apple in a steamer basket over boiling water.
Cover and steam over medium-high heat for 5-7 minutes, or until the parsnips are fork-tender.
Carefully remove the steamer basket.
Reserve 1/3 cup / 85 mL of the steaming liquid before draining the rest.
Transfer the steamed parsnips and apple to a food processor.
Add the reserved cooking liquid, oil, nutmeg, lemon peel, salt, and white pepper.
Process on high until the mixture is completely smooth and creamy.
Transfer the purée to a warm serving bowl.
Stir in the chopped toasted walnuts and fresh parsley just before serving.
Roasting the parsnips and apple at 400°F / 200°C until tender will create a deeper, more caramelized flavor than steaming.