Add a large casserole dish onto a high heat and add a little olive oil to help with browning the beef.
Season the brisket well with salt and pepper making sure all sides and the ends are seasoned.
Place the brisket in the hot pan turning regularly so it browns all over.
When browned all over, turn the heat off and remove the brisket to a plate to one side for a moment.
Add the onions to the bottom of the pan and place the brisket (fast side up) on top. You have created a little trivet.
Smear the 2 teaspoons of English Mustard of the top of the brisket and add the rosemary leaves on top of the mustard.
I now put a piece of foil over the top of the casserole dish and then place the lid on top to create a good seal.
Place it in the oven for between 4 hours, if you are carving, or 5 hours if you are pulling.
After about 2 hours, remove the lid and foil and check. There should be moisture in the dish but if not, add a little water to the pan and reseal. Pace back in the oven and cook for the remaining period.
When cooked, remove the brisket and place on a large chopping board/platter and carve/pull apart. If you only use half now, there is plenty of leftovers for other meals and other days.
Serve with potatoes and seasonal vegetables as the first or several dishes.