Celery root (celeriac) is a knobby, homely-looking root vegetable with mild celery flavor that pairs deliciously with apple and potato. Cutting the dense-textured celery root into smaller pieces than the potato ensures both finish cooking at the same time. Use any variety of apple for this recipe. I picked up the trick of simply mashing the potatoes and other veggies with a whisk from Jennifer Shaertl’s clever book, Gourmet Meals in Crappy Little Kitchens. Serve with our Spiced Pork Roast, Simplest Roast Chicken or Crispy Duck Breasts with Maple-Bourbon Sauce.