Either peel the cucumber or just wash it well and then cut it long-ways down the middle. Scoop out the seeds using a teaspoon then cut the flesh into slices about 1" (2.5cm) thick. Bring a large pan of water to the boil and add the vinegar, thr salt and the sliced cucumber. Boil gently for 10 minutes then drain well in a colander.
While the cucumber is cooking, squeeze the juice from the lemon. Place the cooked cucumber in a serving dish then add the butter and lemon juice. Sprinkle the chopped parsley over the top.