Advice on narrowing a window recess before tiling

[Ricardus]

I'm tiling into a window recess in my kitchen and I want to narrow the recess about 5mm on one side before tiling over with 10cm tiles. (So the surface of the tiles are flush with a kitchen unit that overhangs the recess)

As my plastering skills are comparable to those of Stephen Hawking, I thought of putting a strip of hardboard up and tiling over that.
Or just putting the adhesive on a lot deeper than usual and supporting the tiles with a board until the glue dries.

 

[jefoss]

if its only 5mm id pack up with the adhesive i was using to make it level dude

bear in mind tho that if your using extra adh allow time for it to dry before grouting