Slice the onions and spread over the bottom of the slow cooker along with the peeled garlic cloves.
In a bowl, mix the meat rub spices together and with your kitchen gloves on, generously rub the mixture into the meat all over. Place the brisket on top of the onions and garlic, fat side up if your brisket has a side with fat on it. Pour any remaining spices on top of the brisket.
In the same bowl as the spices were in (unwashed), mix together the stock, tamari sauce, whole grain mustard and Worcestershire sauce. Pour this mixture down the side of the slow cooker to avoid washing the spices off the meat. The liquid won't fully submerge the meat and that's ok.
OPTIONAL: At this stage, you could pop the lid on the slow cooker and put it in the fridge to marinate for 6-8 hours or overnight before continuing with the next step.
Cook on low heat for 8-10 hours ( I usually do 10 hours), or until meat is 90.5°C-96°C.
Carefully transfer the brisket to a baking tray lined with baking paper or onto a large platter and allow to rest for 20 minutes before slicing or shredding. It should pull apart very easily.
Whilst the meat is resting, put the caramelised onions and garlic from the crockpot aside to serve with the meal and strain the liquid from the slow cooker into a pot. Add in the flour. Whilst stirring, bring to a simmer until it becomes a thickened gravy.
Serve with the caramelised onions and garlic, gravy, mashed potatoes and peas.
Enjoy!