Spray slow cooker with cooking spray or use a disposable liner. Place onions in bottom of slower cooker.
Place brisket on top of onions and sprinkle onion soup mix over top.
In a small bowl, whisk together barbecue sauce, apple cider, and liquid smoke. Pour over brisket.
Cover and cook on low for 8-10 hours.
Remove brisket to a cutting board and allow to rest for about 10 minutes. Slice meat thinly against the grain. Spoon sauce and onions from pot over the meat. Serve on buns as sandwiches, if desired.
Notes
Tip: the leftover meat freezes really well for a later meal! Reheat thawed leftover meat and additional barbecue sauce in a saucepan over low heat until warm.