Chop the onion finely and carefully mash or press it through a garlic press or blender. Knead the mayonnaise with the onion, add the tomato paste and salt it to taste.
Optionally, add a little bit of very finely chopped parsley. Mix well again, to obtain a homogeneous mixture or blend it with a blender. To obtain a light sauce, add the yoghurt, but be careful for it not to become too runny.