1.Preheat oven to 220°C/425°F. Line two large shallow roasting pans with baking paper.
2.Combine oregano, paprika, chilli, garlic and half the oil in a large bowl; season well. Add chicken and celeriac; mix well to coat evenly. Divide mixture between the lined pans, placing chicken skin-side up. Roast for 40 minutes or until chicken is golden brown.
3.Meanwhile, heat remaining oil in a large saucepan over low-medium heat. Coarsely chop parsley stems to yield ¼ cup; refrigerate leaves. Cook onion, celery, fennel and parsley stems, stirring, for 10 minutes or until softened. Add chicken and celeriac mixture, including the roasting juices. Squeeze garlic from skins; add to pan. Discard skins.
4.Add stock to saucepan; bring to the boil. Stir in barley. Reduce heat to low; cook, covered with a tight-fitting lid, stirring occasionally, for 40 minutes or until chicken and vegetables are tender. Remove chicken from pan; stand until cool enough to handle. Break chicken into chunks; discard skin and bones. Skim fat from surface of soup. Return chicken to pan with cavolo nero. Simmer for 10 minutes; season to taste.
5.Finely chop half the reserved parsley leaves; stir into soup. Serve half the soup among four bowls; top evenly with parmesan and remaining parsley leaves. Transfer remaining soup to an airtight container; cool, then store
For less heat, reduce the chilli flakes to 1 teaspoon. Black barley is available from some health food stores, delis and gourmet food stores. You can use pearl barley instead.
Refrigerate soup for up to 2 days. Freeze for up to 3 months; thaw in the fridge, then reheat in a microwave. The barley will thicken the mixture on standing; add a little extra stock or water when reheating.