Wash and dry the cucumber, cut in half lengthways and remove the seeds with a teaspoon. Cut the cucumber into thin slices, season with salt and leave to stand for about 15 minutes.
For the dressing, mash the avocado together with the lime juice with a fork. Stir in yogurt, garlic, chilli and pepper. Dilute with milk or water and the resulting cucumber juice to the desired consistency, fold in the dill, mix with the cucumbers, season to taste.