First, to make the Tiger's Milk put the fresh ginger, garlic, coriander stalks, red chilli, and the lime juice in a bowl. Stir and then leave to infuse for 5 minutes. Strain the mixture through a sieve into another bowl. Add salt and the onion and mix well.
Rinse the onion and then leave it to soak in iced water for 10 minutes. Drain thoroughly, spread out on a paper towel or a clean kitchen towel to remove any excess water and then place in the fridge until needed. This will reduce the strength of the onion and help to keep the slices crisp.
Cut the fish into uniform strips of around 1 1/4 by 3/4 inch (3 by 2 cm). Place in a large bowl, add a good pinch of salt, and mix together gently with a metal spoon. The salt will help open the fish’s pores. Leave this for 2 minutes and then pour over the tiger’s milk and combine gently with the spoon. Leave the fish to “cook” in this marinade for 2 minutes.