- 3 pounds celery root, peeled, cut into 1/2″ cubes
- 3 pounds russet potatoes, peeled, cut into 1″ cubes
- Kosher salt
- 2/3 cup heavy cream
- 2/3 cup whole milk
- 1/2 cup (1 stick) chilled unsalted butter, cut into 1/2″ cubes
- Freshly ground black pepper
- 1 tablespoon good-quality hazelnut oil
- 1/4 cup coarsely chopped toasted hazelnuts
- Special equipment: A food mill or potato ricer
- Place celery root and potatoes in a large heavy pot. Add cold water to cover; season with salt. Bring to a boil over medium-high heat. Reduce heat; simmer until vegetables can be pierced with a fork, 20-25 minutes.
- Drain vegetables well. Pass vegetables through food mill or potato ricer into same pot.
- Meanwhile, bring cream and milk just to a simmer in a medium saucepan; remove from heat. Stir vegetable purée constantly over medium-low heat until excess moisture evaporates, about 2 minutes.
- Stir butter into purée a few pieces at a time, blending between additions. Remove from heat; slowly fold in cream mixture until a smooth purée forms. Season to taste with salt and pepper. DO AHEAD: Purée can be made 1 day ahead. Cover; chill. Rewarm in a microwave or in a heatproof bowl set over a large pot of simmering water.
- Strain hot purée through a medium-mesh sieve into a serving bowl (this makes the texture especially smooth and creamy). Drizzle with oil; scatter hazelnuts over.