The steel lintel above the window has wood stuck or screwed to it to proved a surface that can be plasterboarded. The wood will be useful if a curtain rail needs to be screwed in as there will be something to get the screws into.
This galvanised steel lintel has plywood stuck to it with Gripfill. I tried self tapping screws, but they kept snapping; the metal is too thick really. Gripfill is bloody marvelous in this situation, the bond is really strong.