Heat oil in a large frying pan. Add onion, ginger/garlic paste, chillies, ground coriander and garam masala, and fry until onions turn golden brown. Add chicken pieces, turmeric, nutmeg and chilli powder and let cook for 5 to 6 minutes. Add salt and additional coriander powder to taste and cook until done. Garnish with coriander if desired, and serve.