For the last half-hour, check every 10 minutes. The wine and onions will be almost a jelly at this point and will badly want to scorch. Stir well and add a bit more wine if necessary. Whenever I've written about high-heat braising, I've tried to emphasize the importance of using the right pot. Cast iron is a very poor conductor.
Size counts too.
The pan should be just big enough to hold the meat comfortably without it touching the sides. The bigger the pan, the more liquid you'll have to add to come to the same level on the meat. I've got another cast-iron braising pan that is larger and oblong. In testing the gratin, we ran into another pan oddity. My gratin dishes are old enameled cast iron and that didn't happen.