Cut the cucumber into small cubes and season well with salt. Let it steep for 15 minutes. In the meantime, cut the avocado and feta cheese into small cubes as well.
For the dressing, mix sour cream, mustard, vinegar, a pinch of salt (not too much, as the cucumber and feta cheese are already salted), pepper, dill and a hint of curry.
Pour off the cucumber water, mix in the avocado and feta cheese and add the dressing.