Preheat oven to 160°C (325°F). Heat a large heavy-based saucepan (make sure it has a lid!) over a medium-high heat and add the olive oil. I used my 28 inch Le Creuset pot for this, which is perfect.
Season the brisket with salt and pepper, then when the pan is hot enough, brown the meat on all sides finishing with the fat side up.
Switch off the heat and spread the mustard over the top (fatty side) of the brisket. Lay the rosemary sprig on top then add a little more salt and pepper
Pour the apple cider vinegar and 2 cups of the broth into the pot around the sides of the meat. It should come roughly half way up the side of the brisket so pour slowly as you may need a little less or perhaps more depending on the shape and size of your meat and the size of the pot.
Put the lid on your pot and place it into the oven to cook for 3 hours. At about the 2 hour mark check there is sufficient liquid left in the pot. If not add an additional half cup. You don't want the meat swimming in broth when it's finished cooking but having a little left to toss the meat in makes it extra juicy.
Once cooked remove from the oven, shred with two forks and toss through the juices left in the pan. Serve in a Buddha bowls, tacos or with some roast veg.