Place potatoes and ½ tsp of the salt in a pot with water. Bring to the boil and cook until tender. Allow to cool, peel and cut in cubes.
Mix mustard, the rest of the salt, lemon juice, pepper, mayonnaise, yoghurt and parsley.
Mix sauce into potatoes with red pepper and onion.
Sprinkle with egg and serve as a side dish.
To soften the strong flavour of raw onion, cover with boiling water and allow to stand for a few minutes. Drain well and use as above.
If fresh chives are available, add 2 tbsp chopped chives to the sauce.