Place onion, celery, spices and a pinch of salt into a small saucepan. Place the chicken, skin down, and fill with enough water to cover. Bring to boil then lower heat and simmer gently for 6 minutes.
Turn off heat and leave the chicken for 20 minutes in hot liquid. Remove and when cooled, shred chicken into large bite-sized pieces. Discard skin and bones. Strain stock through muslin and reserve.
To make the rice, heat oil and butter in a large saucepan and sauté onion with a tsp of salt on low heat until it softens, about 10 minutes.
Increase heat, add lamb and sauté until all juices have evaporated. Add spices and rice and stir through for a few minutes to coat the rice. Pour in 1 1/4 cups of reserved chicken stock. Bring to boil, then lower heat, cover pan and simmer gently for about 10 or 12 minutes until rice is al dente.
Serve rice and shredded chicken garnished with roasted nuts and coriander.