So are you doing two flats? Are you going to sear first then braise with some carmelized onions and beef stock -something like that. I usually wrap the roasting pan with plastic wrap then foil. Start checking after 3 hours but you really have to check and give yourself enough wiggle room if it takes longer
That’s pretty much where I was going. I should have mentioned that this will be done in two separate baking dishes, at the same time, on the same rack Not searing, but that is a thought.
This may be way out in left field, but, if you are searing, would you consider doing a dry brine brine first? I’m just babbling on this point.
Can you expand on the benefit of plastic wrap and foil? That’s a new one on me.
Don’t think you need to brine personally. Plastic wrap really helps keep the moisture in, foil keeps it away from direct heat. I would go sear then 300 but won’t argue if you want to go slower, can’t hurt