Celery Root Purée with Toasted Hazelnuts

Name: Celery Root Purée with Toasted Hazelnuts
Ingredients: 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; A food mill or potato ricer