1) Mix the cucumbers, red onions, and 1 1/2 tbsp of salt in a bowl. Pour them into a colander and suspend it over a bowl. Wrap the bowl and colander with cling film, and place in the refrigerator to drain for at least 4 hrs or overnight. Discard the liquid that collects in the bowl.
2) Pour the yoghurt into a sieve lined with a paper towel and suspend it over another bowl. Wrap the bowl and sieve in cling film and refrigerate for at least 4 hrs or overnight. Discard the liquid that collects in the second bowl.
3) When the cucumbers are ready, roll them up in paper towels or a kitchen towel and press the towel lightly to remove most of the liquid. Place the cucumbers and yoghurt in a large bowl with the sour cream, vinegar, dill, 2 tsp salt, and the pepper. Toss well and refrigerate for a few hours to allow the flavours to blend. Sprinkle with 1/2 tsp salt and 1/2 tsp pepper and serve chilled.