Celery root (along with a bit of grated Grana Padano cheese) adds a deep, earthy backbone to this potato mash. It's a tweak even a diehard traditionalist can love.
- 1 lb. celery root, peeled and cut into ½-inch cubes
- 2 cups half-and-half
- 8 Tbsp. unsalted butter
- 8 sprigs thyme
- 3 lb. russet potatoes, peeled and cut into ½-inch cubes
- 2 tsp. kosher salt, plus more to taste
- 1 cup finely grated Grana Padano cheese
- Freshly ground white pepper
In a 2-quart saucepan, combine the celery root with the half-and-half, butter, and thyme; bring to boil over medium-high heat. Lower the heat to medium-low and simmer, partially covered, until the celery root is tender, about 25 minutes.
Discard the thyme. Transfer the celery root mixture to a food processor and purée until smooth. Scrape through a fine-mesh sieve into a clean saucepan; keep warm over low heat.
Meanwhile, in a 6-quart saucepan, add the potatoes, salt, and enough water to cover the potatoes by 1 inch. Bring to a boil over high heat, then lower the heat to medium-low and simmer until the potatoes are tender, 10–15 minutes. Drain the potatoes, then pass through a ricer into a large bowl. (For even smoother potatoes, mash with a potato masher as well.)
Transfer the potatoes to the saucepan with the celery root and whisk in the cheese until the cheese is melted and the potatoes are smooth and hot. Season with salt and pepper to taste.