Place all of the ingredients, except the fish sauce, in a small deep-sided saucepan and bring to a gentle simmer. Simmer for 15 minutes until the ‘raw’ taste of the sea urchin has been cooked out. Remove from the heat and season with the fish sauce. Using a hand-held stick blender, blend until smooth, then pass through a fine sieve.
When ready to serve, froth with the hand-held stick blender.
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