Remove brisket from fridge, discard packing and allow to come to room temperature for 30 mins.
Place the sliced onions and sprigs of rosemary in the base of a slow cooker (Crockpot).
Place brisket on top or onions, season with a little salt, a generous grinding of pepper and crumble over beef stock cube.
Set the slow cooker to high for 4 – 5 hours or low for 7 – 8 hours.
Once the beef brisket is cooked and tender (see advice above for checking doneness above), remove from slow cooker, cover with foil and allow to rest for 10 minutes.
Meanwhile prepare the onion gravy. Remove the rosemary sprigs (if using) and pour the meat juices, onions and garlic into a pan.
Mix 2 teaspoons or cornflour with 2 teaspoons of cold water to form a thin paste and stir into the pan.
Slowly bring gravy to the boil whilst stirring with a wooden spoon and then simmer until the thickens (you may need to add a some boiling water to get the desired consistency/quantity).
Once the meat has rested, carve across the grain and serve with the onion gravy and your choice of vegetables.