Filling gaps around doors

[Alex-m86]

What’s the best way to fill these large gaps around doors before plastering? Trying to avoid cracking as much as possible but appreciate some might be inevitable. Thanks

 

[ktuludays]

Plasterboard

 

[Wayners]

I'd put a reinforced concrete lintel above door and brick up. Only £20.plus a few bricks
That sorts the top.

Bit more wood and plasterboard up the edge.

Leaving ceiling?

 

[RandomGrinch]

Something I wish I did when I had the opportunity - it looks like you've taken down the lath and plaster on the left hand wall; will there be any natural light entering the stairwell, when the wall is reinstated?
If not, you could even consider extending the gap above the door and adding a top light?