Drop the prawns into a small saucepan of boiling salted water. Bring back to the boil, simmer 2 minutes or until the prawns are just cooked. Remove from the heat, drain and cool. Slice the prawns thinly lengthways. Reserve.
Melt the butter in a large saucepan over moderate heat and add the onion, garlic and zest. Fry gently, without browning for 10 minutes until the onion is soft. Add the stock and diced potato and bring to the boil. Simmer 10 minutes until the potato is soft. Add the peas and boil gently for 5 minutes. Remove from the heat and purée using a wand blender or food processor.
Pour back into a clean saucepan and add the cream. Bring to the boil, taste and season. To serve, pour the soup into bowls, top with the cooked prawns and garnish with mint leaves.
Defrost your prawns quickly by placing in a colander and running them under cold water.