Put the egg yolk, vinegar and mustard in a bowl. Season with salt and pepper. Whisk with the whisk of the hand mixer for about 1 minute. First add the rapeseed oil drop by drop (approx. 5 tbsp.), then slowly add the rapeseed oil in a thin stream, stirring continuously. Fold in the soured milk. Season mayonnaise to taste with salt and pepper. Put it in a cold place
Clean and trim the mushrooms. Halve mushrooms. Cut oyster mushrooms into small pieces according to size. Clean and wash spring onions and cut into fine rings. Put aside about 1/3 of the spring onions.
Heat olive oil in portions, fry the mushrooms in it in portions. Put all mushrooms back into the pan, season with salt and pepper. Add spring onions and fry briefly. Sprinkle with onions and serve with mayonnaise. Serve with a grain baguette