Cut the cucumber in half lengthways, scoop out and discard the seeds and cut the flesh into 1 cm (½ inch) slices. Put in a glass or ceramic bowl. In a small bowl, put the salt, vinegar, sugar, chilli and ginger, add the water and mix well. Pour over the cucumber, cover and leave to marinate at room temperature while you cook the trout.
Brush the trout with the oil and season to taste with crushed Szechuan pepper and salt. Place the trout in a single layer on a grill rack and grill for 5–6 minutes on each side or until cooked through. Leave to rest for a few moments, then garnish with chopped chives and serve with the cucumber relish and lemon wedges.