Peel the garlic and press through. Put egg yolk, vinegar and mustard into a tall mixing bowl. Whip with a hand blender at the highest setting. First add the rapeseed oil drop by drop (approx. 5 tbsp.), then pour in a thin stream slowly, beating continuously, until a thick mayonnaise is obtained.
Fold in garlic and soured milk. Season cream to taste with salt and pepper and chill.
Clean the mushrooms, possibly wash or rub with kitchen paper. Cut smaller depending on size. Clean and wash spring onions and cut into fine rings.
Heat olive oil in portions in a large frying pan and fry the mushrooms. Season with salt and pepper. Put all mushrooms back into the pan. Add 2⁄3 of the spring onions and fry briefly. Sprinkle with the remaining spring onions.
Arrange mushrooms, add garlic cream. Wholemeal baguettes are delicious with it.