"Hot and sour soups come in all varieties and hues all over Thailand," says David Thompson. "This one is golden and sour and sharp. It's currently my favourite soup - a panacea for all that ails. When you add the fish - any type - leave it to boil for a moment and do not stir until the fish is sealed and slightly cooked. This prevents the soup from tasting too fishy. Season the soup in the serving bowl so the lime retains its pert sourness, making the soup even more comforting."