Help! Tough brisket!

My brisket also took longer to cook than usual for some reason. I ended up leaving it in the oven for at least an additional 2 hours, then turned the oven off and just let it sit. It was delicious!

In addition to cooking time, first cut brisket has little fat. though I'm always looking out for ways to make dishes more healthy, the brisket often does better with a slightly fattier second cut. Also much cheaper!

In order to really break down the collagen the temp of the meat needs to be around 200º F for an hour or so. I echo the longer cook time advice.

I also suspect you didn't cook it for long enough. I cook my brisket (in a chili sauce with lots of onions) for 4-5 hours at 350 and it is so tender it almost falls apart. Also, with brisket it is important to slice it against the grain to maximize tenderness.

Your braising liquid sounds lovely, and you seared it beautifully for not too long. Next time, though, I'd suggest you set it in the oven at 300-325 for a longer period of time. A braise can't really be rushed without sort of the results your's achieved. But have hope! Keep all that lovely braising liquid and return the leftovers to the pot after both have cooled. Refrigerate overnight, then raise to a good simmer on the stovetop and re-braise in the oven at 325 until time to serve. Give it a good couple of hours more, and I suspect you'll be at the silky tender stage.

This might be something that some purist wouldn't like.

But I use a "needle tenderizer".
It breaks down tough tissues in meats like briskets and helps with marinading. It works for me for those cuts of meats. You can find them online or in some supermarkets 20 bucks or less.