Put the onions into the bottom of the slow cooker and sit the brisket on top (if it is rolled with string, leave this on).
In a bowl, mix the sauce ingredients together. Add the beef to the bowl of spice paste and roll it around so all sides are covered. (It is easiest to do this using clean hands).
Put the lid on and cook on HIGH for 5-6 hours or LOW for 7-8 until very tender.
When the meat is cooked and looks like it will pull apart, remove any string, slice thinly and serve with the onions.
OPTIONAL: Once you've removed the meat from the liquid, you can reduce the sauce to concentrate the flavour if you'd like to. Put the cooking liquid into a small saucepan and put it on the hob over a medium high heat. Let it bubble for 5-10 minutes until it is thickened. Then pour back over the meat as above.
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