Celery root (also known as celeriac) and apple are made for each other. This sweet, earthy purée is a simply stunning accompaniment to roast pork, turkey or duck. Take a break from mashed potatoes and serve this delectably creamy side dish for an intriguing change of pace.
Melt the butter in a large saucepan and add the celery root cubes. Cook over medium heat, stirring frequently, until the celery root starts to soften, about 15 minutes. Add the apples, salt, nutmeg, star anise and apple cider, and bring the mixture to a simmer. Cover the pan and cook slowly until the mixture is very soft and tender, 45 to 60 minutes. Uncover and add the heavy cream. Cook until the cream is almost completely absorbed, about 10 minutes.
Remove the pan from the heat and discard the star anise pod. Purée the mixture with an immersion or stick blender until it is smooth, or transfer the mixture to a food processor and process until smooth.
The mash can be made a day in advance of serving. Cool completely and refrigerate, covered. To reheat, place in a small saucepan and cook over low heat, stirring frequently, until hot, 10 to 15 minutes.