Roughly chop onions
Place the onions in slow cooker, with the brisket on top. Cover with chicken stock.
Cook on high for 6-8hrs
To make the gravy, remove the cooking liquid & onions from the slower cooker. Skim off the fat from the top of this liquid.
Heat the butter in a sauce pan until bubbling, add the flour & cook out on medium heat for 1-2min, stirring constantly.
Add the cooking liquid to the sauce pan gradually, stirring constantly to keep a smooth consistency. You may need to use a whisk during this process.
Add the crushed Oxo stock cube to the saucepan & whisk to combine
Cook on low-medium heat until desired consistency is achieved (mine was perfect after 2-3 minutes)
Add the gravy back to the slow cook until ready to serve.