Peel the cucumber, cut it in half lengthwise and remove the seeds. Cut the cucumber halves into slices 1 cm thick. Peel, halve and slice the onion. Put the cucumber, onion and parsley in a serving bowl.
Mix together the lemon juice, vinegar and syrup and pour over the vegetables.
Marinate for at least 1 hour before serving.
Serve with grilled meat, ideally strongly spiced.