Place the shallots, ginger, chilli, rice wine vinegar and sesame oil in a saucepan and heat gently until reduced by one-third. Remove from the heat, stir in the sugar and soy sauce, and set aside to cool
Stir the pickling juice, fish sauce and lime juice into the mixture until well combined.
Heat 1cm/½in of the oil in a large, deep-sided frying pan until a breadcrumb sizzles and turns golden-brown when dropped into it. (Caution: hot oil can be dangerous. Do not leave unattended.) Add the sliced shallots and shallow fry, stirring now and then, until crisp and richly golden-brown. Remove the onions from the pan with a slotted spoon and set aside to drain on plenty of kitchen paper.
To serve, spoon the vinaigrette over the oysters in their shells and top with fresh coriander and crispy shallot rings. Serve over crushed ice.