Window fill advice needed

A search on "windows" brings up much info! Wondering if someone can point me towards the best way to deal with this one. (see attached pic below) Strange.........all this time on this forum and this issue doesn't stick in my head like many of the others. Also I should mention that this will be a "typical" (notice the quotes!) 'box within a box' type-build. (I.e. the outer walls will be "Sharwarded"/beefed-up). This window is one of 4 windows located on the outer-most, beefed-up leaf. (Note: The wife wants to put some sort of decorative shutter in there that will still be visible from the outside....I know, I know). Thanks for any advice.