Put the yoghurt in a bowl and stir in the salt and mustard (use very simple tube mustard, medium hot). Then add the sugar to taste - it should taste sweet.
Now add the finely chopped onion and season with salt, sugar and mustard until the desired flavor is achieved.
For a bean salad, mix the dressing with the (green) beans and let it steep for a while (at least 2 hours).
For iceberg lettuce, mix the dressing and lettuce just before serving.