Prepare brisket rub: Mix paprika, pepper, salt, brown sugar, chili powder, and cumin together in a bowl; rub evenly over brisket. Put brisket in a large, resealable plastic bag; refrigerate for 30 minutes to overnight.
When ready to cook, prepare barbeque sauce: Stir barbeque sauce, water, Worcestershire sauce, and liquid smoke together in the bottom of a slow cooker.
Remove brisket from the refrigerator and take out of the bag. Lay brisket in sauce in the slow cooker and arrange onions over top.
Cook on Low until brisket is very tender, 6 to 7 hours.
Rest brisket 10 minutes before slicing or shredding; serve with sauce from the slow cooker.