The first step calls for putting onions over the brisket. How much? Onions are not listed in the ingredient list.

I'd thinly slice (north-pole-to-south-pole) two medium onions, but one large one, or even two small ones, should work as well. You're just trying to get more flavor into the braising liquid and therefore into the meat itself. Sounds delish, by the way! ;o)