Everyone has got it nailed... flycut and/or machine the raised side first to level it out... then the cupped side.
Just a note about your mill... if you don't have a collet to fit your fly cutter you can use a 3 or 4 flut endmill that is quite wide. like 3/4" or 1" and do several passes with your table. Just lock the mill head at the height you want to take off a few thousandths and make 2 or 3 passes. Then lower it another few thousandths if you haven't gotten it all.
Finally ... lap it for a nice finish.
I'm not completely familiar with machining terms... but I have a mill (in storage..
) that I used on my blocks.
The raw copper I got came with the same slight bow to it and that's how I corrected mine.