Peel the onions, peel the apple and remove the core. Cut the onions, apples and pickles into large pieces. Wash the parsley and shake very well dry. Remove the stem and cut roughly. Put in a blender / hand blender / food processor and chop very finely, but do not puree.
Mix the mayonnaise with the yogurt and mustard and season with salt and pepper. Pour the mixed ingredients into the mayonnaise, season to taste again and let steep for 30 minutes or more.