Finely chop the onion and soften in a pan with oil until soft. Don’t let them brown. Add a little water then stir in the mayonnaise (start with a couple of tablespoons – you can always add more). Add a dash or two of vinegar and a blob of mustard, sweetener and salt & pepper.

Halve or quarter the boiled potatoes (leaving the skins on for added texture and taste) and mix into the onion & mayo mixture. If the sauce is too thick, just add a little more water or vinegar to adjust.

For extra creaminess use some a couple of tablespoons crème fraiche or yoghurt.