SERVES 8

Celery root (along with a bit of grated grana padano cheese) adds a deep, earthy backbone to this potato recipe. It's a tweak even a diehard traditionalist can love.

1 lb. celery root
2 cups half-and-half
8 tbsp. unsalted butter
8 sprigs thyme
3 lbs. russet potatoes, peeled
   and cut into 1⁄2" cubes
2 tsp. kosher salt, plus more for seasoning
1 cup finely grated grana padano cheese
Freshly ground white pepper

1. Peel and cut celery root cut into 1⁄2" cubes and combine in a 2-qt. saucepan with half-and-half, butter, and thyme and bring to boil over medium-high heat. Lower heat to medium-low and simmer, partially covered, until celery root is tender, about 25 minutes.

2. Discard thyme. Transfer celery root mixture to a food processor and purée. Scrape mixture through a sieve into a clean saucepan; keep warm over low heat.

3. Meanwhile, combine potatoes and 2 tsp. of the salt in a 6-qt. saucepan and add water so that potatoes are covered by 1". Bring to a boil over high heat, lower the heat to medium low, and simmer until potatoes are tender, 10–15 minutes. Drain potatoes and pass them through a ricer into a large bowl. (For even smoother potatoes, mash with a potato masher as well).

4. Transfer potatoes to the saucepan of puréed celery root and whisk in cheese until cheese is melted and potatoes are smooth and hot. Season with salt and pepper to taste.