4 pounds Yukon Gold potatoes, peeled
1 medium celery root (about 1 pound), trimmed and peeled
2 tablespoons salt
Freshly ground pepper
1/2 cup extra-virgin olive oil
Regular or nondairy sour cream, for serving
Shred potatoes and celery root on the large holes of a box grater. Working in batches, wrap in cheesecloth and squeeze out liquid. Toss with salt in a medium bowl. Season with pepper.
Preheat oven to 400 degrees. Heat 3 tablespoons oil in a 12-inch ovenproof nonstick skillet over medium-low heat. Spread shredded potato-celery root mixture evenly in skillet; press gently to flatten using a spatula. Cook for 10 minutes.
Run spatula around edge to loosen; spoon 2 tablespoons oil around edge. Cook until underside is golden and begins to crisp, 10 to 15 minutes more. Run spatula around edge to loosen; invert onto a plate.
Add remaining 3 tablespoons oil to skillet. Return rosti to skillet, golden side up; press gently to flatten. Cook, shaking occasionally to loosen, until underside is golden and begins to crisp, about 20 minutes.
Transfer skillet to oven. Bake until cooked through, 10 to 15 minutes.
Return to plate; cut into wedges. Serve with sour cream.