Clean and slice the mushrooms and fry in a pan. Cut the hard-boiled eggs into slices and halve the slices again. Put together with the mushrooms in a bowl. Dice the onions and add them.
Mix with the mayonnaise (I always use Miracel Whip Balance - it`s not so greasy and not too sour) and sprinkle with the chives. Stir everything again and then ideally let it steep overnight.
IMPORTANT: Even if the egg salad seems a little too firm, do not add any milk. This will make the salad too runny. The mushrooms have enough liquid. (I always pour off some of the liquid after frying!)