Put the sugar in a large shallow pan, along with a small splash of water. Heat gently, swirling the pan, until the sugar has dissolved. Increase the heat and bubble the syrup until it turns a dark amber colour. Add the fish sauce, most of the chilli and ginger, then splash in 1 tbsp water to dilute. Boil again until syrupy, then add the fish fillets, skin-side down, and the bok choi, cut-side down.