1) Cook the prawns in boiling water for three minutes, drain and cool in the refrigerator.
2) Put the mint and lemon juice in a food processor and pulse to coarsely chop the mint. Drizzle the olive oil into the processor while it is running, until the mint is finely chopped.
3) In a serving bowl, toss together the prawns, cucumber, mint mixture, zest, salt and pepper. This will keep in the refrigerator in an airtight container for up to two days.