best way to make up a return into a doorway thats oversize?

Hi Guys' I wonder if you could comment on the following.. The builders cut a new doorway into my new extension going back a few months, however they had a few goes at getting the cut straight and in the right place in the inner skin to match up the the cut in the outer skin... I'm now at a point where I'm looking to finish this area and I find myself with a return into the doorway, about 8" wide (to close the gaps in what was the inner and outer walls) but about 40-50mm deep per side to get back to where the plaster needs to be and to match up onto the frame for the door.. so what do I do?... If the plasterboard is 12mm, do I put 28-38mm of dot and dab on the back of the plasterboard to make it right?... Do I dot and dab some plasterbooard to the return and then dot and dab another on top to make it right?.. or do I screw some wood in and dot and dab to that?....
any comments appreciated

A photo would be very handy Beaver.

maybe over complicating the description...
and for some reason I can't upload a pic??....

anyway.. its a door cut through what was the outer skin of the house.
Ive put a 30" frame into what was the outside brick wall, but the inside block wall has been cut out much bigger. I now need to make up the differance to bring the inside return back to match the frame... It's about 40 / 50mm a side that needs making up.

can you 50mm of dot and dab on?.. or is that too much.. and if so, whats the best way to build it up?..

Anyone? Can you dot and dab up to 50mm deep behind the board? Cheers

Too much depth IMHO. If it were me, I'd be inclined to dot and dab some stud timber into place with mechanical fixings (ie screws), let it go off and re-tighten the screws before screwing plasterboard and skim on top of this.

Maybe overkill, but since doors tend to exert torque on the surrounding areas, I'd rather do this that have hairline cracks appear in a few months.

okay, thanks for that. TBH I've got two bits of stud timber sat in there now but didn't know if the dot and dab stuff would be okay with it?.. so I'll give it a go... If I can't get the timber level.. is it okay to dot / dab onto that with the top layer of plaster board?... Thanks