Preheat oven to 350 F/175 C
Season the brisket with salt and pepper. Heat a frying pan/dutch oven on high heat. Add oil and sear brisket on all sides until nicely browned (8 minutes). Set aside.
In the same pan add olive oil. Sauté onions and celery on medium heat till soft and golden brown (10 mins). Add garlic and mushrooms and sweat till water cooks off and mushrooms are wilted.
Add cranberry sauce, balsamic vinegar, and sweet chilli sauce to a bowl. Mix the chicken stock powder in boiling water to dissolve. Add to the bowl and whisk to combine.
Pour veggies into the bottom of an oven-proof baking dish/dutch oven. Place the seared brisket on top. Pour sauce over the meat. Tie the rosemary and thyme into a bundle and add along with bay leaves to the roasting pan.
Cover with lid or parchment paper and foil. Seal well to create a steam bath.
Roast on the middle rack of oven roast for 2 ½ hours turning halfway through. The brisket should be fork tender. (If meat resists cook for another ½ hour). Take the meat out and cover with foil until cool. *****
If the sauce is watery cook off the excess liquid on the stovetop till it thickens and is soupy.
Slice meat against the grain on the bias. Place back in an oven-proof dish and top with the sauce.
Cover with foil and reheat at 300F/150 C until warmed through (45 minutes). Serve with roasted potatoes and garnish with chopped chives.
*****You can refrigerate the cooled brisket overnight and slice in the morning. Don’t worry if the meat feels hard. It will after being in the refrigerator but it will soften up. Bring to room temperature before reheating.
Make the brisket 2 to 3 days ahead so sauce penetrates the meat. Freeze/refrigerate till ready to use.