1/2tspdried rosemary(or sub 1 tsp Italian spices for both Thyme and Rosemary)
Rinse your brisket if it has seasoning already on it. Heat oil to medium high in a skillet. Brown brisket on all sides, about 3 minutes per side.
Put onions, celery, carrots in bottom of slow cooker. Add the browned brisket.
In a measuring cup mix the stock, tomato paste, vinegar, spices. Pour over the brisket and vegetables. Pour the Guinness over the top of everything.
Cook on low 7-8 hours. During the last hour mix the cornstarch with a few tablespoons of the sauce and shake well in a small covered jar. Pour the slurry back into the slow cooker and allow the gravy to thicken.
Remove brisket from the slow cooker and loosely cover with foil for about 10 minutes before slicing. Serve with the sauce and vegetables.