Rub the roast with the mustard, then season with the rosemary and some salt and pepper. Put the roast on a rack in a roasting pan. Roast for 1 hour.
Reduce the oven temperature to 325 degrees F. Toss the carrots, potatoes, onions and garlic with some oil, salt and pepper and add them to the pan. Continue to roast for 30 minutes.
Add the chicken stock to the pan and continue to roast to the desired tenderness and temperature, 165 to 190 degrees F for brisket. (See Cook's Note.) If you do racks or a leg of lamb, 135 degrees F and up is desired. For a pork rack or loin, cook to 145 degrees F and up. Remove the roast to a cutting board and let rest for 20 minutes.
While the meat is resting, stir the butter into the liquid in the pan with a splash of vinegar. Slice the meat and serve over the vegetables. Top with the sauce.
Brisket is a longer-cooking cut than most. Timing will vary depending on type of meat and size of your roast. For more ingredient substitution ideas, see the article below.