We have installed a complete kitchen unit flush with the wall, including the refrigerator. However, I would not fit a "stand-alone refrigerator" flush with the wall; what if it breaks and you can't get a replacement of the same size to fit the gap? We can replace the built-in refrigerator since only the cabinet frame is enclosed.
Anyway, if you want to do it, the simplest and very flush solution is to make the opening in the masonry larger, put the refrigerator in, and then close the gap with drywall. By the way, letting the refrigerator stick out about two-thirds at the front, as shown in the picture, looks bad in my opinion. For us, only the front panel sticks out so that the cabinets can still be opened, but the drywall is almost flush (1-2mm).
The dark line in the picture is just a shadow; the drywall is really almost flush. I slipped a very thin piece of cardboard between the gypsum fiberboard and the cabinet and then screwed the panels in place.
