Pod the peas into a bowl. Reserve and roughly chop the pods. Peel the prawns, reserving their heads, shells and any roe.
Heat the oil in a saucepan, add the green onions and cook over a gentle heat for 1-2 minutes until soft, then add the garlic, pea pods and prawn shells and cook, stirring frequently, for 4-5 minutes until the prawn shells release a powerful aroma.
Add the ginger, mint, peppercorns and 1 litre water and bring to boil. Skim, reduce the heat and simmer for 15 minutes. Strain the contents of the pan through a large sieve into a deep bowl, pressing down with the back of a spoon to extract as much liquid as possible. Discard the flavourings.
Return the liquid to the rinsed saucepan and add the grated potato and half the peas. Bring to the boil, then cook at a low boil for 5 minutes. Purée, using a hand-held blender. Return to the heat, add the remaining peas and simmer for 3-4 minutes until tender.
Add salt, pepper to taste and sugar, if needed, and transfer the soup to warmed soup plates or bowls. Add the shelled prawns, top with chives and serve.