In a small bowl combine the white vinegar and 1 teaspoon of sugar, stir until sugar has dissolved then add sliced onions. Allow the onions to sit for 15 minutes then drain discarding the liquid.
In a large bowl combine the salt, lime juice and the remaining 1 teaspoon sugar. Stir until sugar has dissolved, add the sardines and allow to marinate for 10 minutes.
Within 10 minutes the sardines should be cured from the acids in the lime juice getting slightly discoloured. Gently squeeze the sardines until there is no juice dripping from them then transfer to another bowl; discard the liquid.
To the bowl with the sardines add the perilla leaves, two kinds of mint, reserved pickled onions, fried Asian shallots, fried garlic chips, garlic oil and dress with dipping fish sauce. Mix well, then transfer to a platter.
Garnish with crushed peanuts and chilli.
Combine the fish sauce, rice vinegar, sugar and 1/2 cup of water in a saucepan and place over medium heat. Stir well and cook until just before boiling point is reached, then remove from the heat and allow to cool. To serve, add the garlic and chilli, then stir in the lime juice. Store tightly sealed in a jar in the fridge, for up to 5 days.
Whiting is a good substitute for sardines.