If your brisket comes rolled (UK), cut the ties and unroll it, then trim off excess fat and season with salt. Heat 1 tbsp of vegetable oil in a frying pan and brown the brisket on both sides, then remove from the pan and put it in the slow cooker.
To the same pan add the chopped onion and sauté over low heat for 5-7 minutes, then top the brisket with the fried onions.
In a medium bowl combine the smoked paprika, dried coriander, cinnamon, garlic powder, salt, canned crushed tomatoes or passata, molasses/black treacle (UK), brown sugar, Worcestershire sauce and wholegrain mustard. Smother the brisket with the spice mix, then flip it over making sure both sides are equally covered, add the bay leaves and cook on high for 6 hours or on slow for 8 hours.
To remove the layer of fat on the surface of the sauce you can either skim it off with a spoon or refrigerate overnight and remove the hardened fat off the surface. The latter is the easier and more effective method but requires an overnight wait time.