Celeriac is the Marmite of vegetables. With its hint of aniseed and rough texture, this root can be divisive but, when cooked, it takes on an irresistible sweetness and a distinctly velvety texture. I love it in this seasonal gratin, which is rich with garlicky mushrooms, melting onions and hearty potatoes. It’s like sinking into a bath of autumn, and just needs a crisp salad on the side.
