Place olive oil and ground sirloin in a large skillet over medium-high heat, browning well, about 5 minutes. (You can also do this right in the crock of many modern slow cookers by using the cooker’s browning function.)
When meat is almost browned, drain off any excess fat, return to heat, and add onions, cooking just until they begin to sweat.
Transfer browned meat and onions to a slow cooker and cover with remaining ingredients, stirring to combine.
Set the cooker to LOW, cover, and let cook 8 hours before serving.
Any kind of ground beef may be used in place of the ground sirloin; just be sure to drain off excess grease.