Trim the chicken of any fat and cut into large chunks. Heat the oil in a large pan, add the sliced onion and cook for 5 minutes until it is softened and slightly golden. Add the curry paste and cook for a minute until all the onion is well coated with the paste. Add the chicken and cook over a high heat until it had lost it's pinky colour, stirring occasionally. Add the coconut milk and bring to the boil stirring to combine all the ingredients. Reduce the heat and simmer for 20-25 minutes until the chicken is tender. Add the fish sauce, lime juice and sugar and stir through the curry. Thicken the curry with the cornflour.Mix the cornstarch with 3 tablespoons of the sauce from the curry to make a smooth paste. Add to the curry and stir through. For the last 5 minutes add the bok choy and cook till wilted and cook.