Preheat oven to 220C. Remove breasts and legs from spatchcock and refrigerate until required. Roast carcass until golden (30 minutes), then transfer to a large saucepan, cover with chicken stock, add fennel seeds and bring to the boil. Skim scum from surface, then reduce heat to medium and simmer until stock is well flavoured (1 hour). Strain and return stock to pan.
Meanwhile, cook barley in boiling water until tender (20-30 minutes). Drain and add to stock.
Heat half the oil in a non-stick frying pan over medium heat, add onion and fennel and stir occasionally until starting to turn golden (15-20 minutes). Transfer to stock and bring to a simmer.
Wipe out pan and heat remaining oil, add spatchcock legs and turn occasionally until golden (3-5 minutes), then add to stock. Add breasts to pan and turn occasionally until golden (3-5 minutes), then add to stock, season to taste and simmer until spatchcock is tender (20-30 minutes).
To serve, divide broth, fennel and spatchcock among bowls and scatter with parmesan, fennel fronds and chilli.