Oh my. Left over? Anyways, slice the brisket to thinness of your liking. Assuming its not already. Place in a pan on the stove top with a bit of your favorite sauce. Finishing sauce, BBQ Sauce, or something like Apple Cider. Basically you want a bit of moisture. Get that going, and cover it.
Meanwhile toast a split loaf of french bread in the oven. Maybe around 250.
When french loaf is nice and toasty, remove from the hot box and top it with your now steaming Brisket, pour on the juices as well. The bread will soak it all up. Top with additional stuff as you like. Onion, Lettuce, Tomato. What ever you like. More sauce if you like.
Now the magic. Close up the sammich, and roll whole thing up in a heavy kitchen towel. Now wait. If you give it about 5 minutes or so, the heat from the meat should slightly steam the bun, along with all the juices soaking into the bun.
Enjoy.