I like to utilize the fat already in the pan from the beef, but if you prefer to use cornstarch, skip the addition of butter and flour in recipe steps 4 through 6 and instead add only 1 tablespoon of butter to the pan for the onions.  Then, simply make a slurry from 1 tablespoon cornstarch and 1 tablespoon cool water and stir it rapidly into the braising liquid over medium heat after you’ve removed the cooked brisket, onions and herbs. It will thicken as it comes to a boil.