Place roast in slow cooker and pierce some holes all over the top surface of it with a sharp knife
Cut large onion into quarters and place around the roast in the base of the slow cooker
Combine rub ingredients and rub onto and into the surface of the roast beef
Squirt the mustard across the top of the roast
Cook on high for 4hrs then low for 2hrs (or Auto for 7hrs or low for 8hrs)
When cooked, using a slotted spoon remove the onion and discard
Remove roast beef from slow cooker and rest it while you make the gravy
To make gravy add the gravy powder sachet (dry, no water added) to the cooking juices and whisk to combine well then turn to HIGH
While carving your meat and plating up your meals have the slow cooker cook on HIGH to thicken your gravy nicely (you could do this on a stove top also if you prefer to do it faster but ours thickened nicely in slow cooker in around 10-15mins)