Pella — 4 Yukon gold potatoes, peeled and cubed
1 1/2 pounds celery root, peeled and cubed
3 tablespoons butter
1/3 cup sour cream or plain yogurt, at room temperature
4 ounces mascarpone, Italian-style cream cheese or conventional cream cheese, at room temperature
1/2 teaspoon salt
freshly ground black pepper
1. Place potatoes and celery root in a large pan; cover with water. Bring to a boil, reduce heat, and simmer until tender, about 20 minutes. Drain, reserving 1 cup water.
2. Add butter and reserved cooking water to potatoes and mash. Add sour cream, mascarpone, salt and pepper, and stir well.