In my Flat the original kitchen layout had a recess (about the size
for a table) which then backed onto the hall cupboard.

The recess and cupboard were knocked together to create a box room so
the kitchen no longer has a recess.
I want to change the layout back to how it was originally - Kitchen
with recess and hall Cupboard. However the original wall between the
cupboard and the recess must have been a supporting wall as there is
an RSJ there.

I need to put up a stud partition wall under the RSJ then plasterboard
over the lot so it just looks like the Original wall.

1) Do I need to build a frame either side of the RSJ?
or
2) can I just build up to the RSJ (if so how would you attach the
ceiling plate to the RSJ)?