Melt the butter in a large saucepan over medium-low heat. Add the onion and sauté until softened but not browned, about 8 minutes.
Add the potato, apples and parsnips; stir in the chicken broth and spices. Increase the heat to bring the soup to a boil, then reduce back to medium-low and simmer, covered, until the vegetables and apples are very soft, about 30 minutes.
Purée in batches, or using an immersion blender. Stir in the cream and season well with salt and pepper.
Notes
Sweet + earthy: Apples balance the earthy parsnips for a subtly sweet, velvety soup.Don't brown the onion: Keep it on medium-low so it softens without colour for a clean flavour.Blend smooth: An immersion blender makes it silky; purée in batches if using a countertop blender (vent the lid).Cream choice: Half-and-half is everyday; heavy cream makes it dinner-party rich.