1. Trim the ends from the cucumber. Cut in half lengthwise. Using a teaspoon, scoop out and discard the seeds, and thinly slice. Cut the bottom 4in (10cm) from each lemongrass stalk, and discard the tops. Discard the tough outer leaves from the lemongrass. Halve the lemongrass lengthwise and smash with a rolling pin.

2. Bring the lemongrass, sugar, and a scant 1 cup of water to a boil in a saucepan over medium heat, stirring to dissolve the sugar. Boil for 1 minute. Remove from the heat and stir in the cucumber. Let cool.

3. Discard the lemongrass. Quarter and core the pears, and slice into thin wedges. Transfer to a serving bowl and add the green and red grapes, cucumber, and syrup. Refrigerate for at least 2 hours to chill. Sprinkle with the mint leaves and serve chilled.