To make the soup, heat the oil in a large saucepan, and quickly fry the tiger prawns for just a minute to half cook them. Remove the prawns from the pan and put aside. In the same pan, add the sliced garlic, ginger and chilli and fry together for 2 minutes. Add in the fish stock and ½ cooked prawns and turn down the heat. Continue to cook the soup for just 3 minutes more. When you are ready to serve the soup, stir in the lime juice and coriander leaves. Divide the bean sprouts equally amongst 2 deep bowls and pour over the hot and sour soup. Sprinkle over the sliced spring onion and serve immediately.