Place beef and onions in slow cooker.
In a small bowl mix together BBQ sauce, balsamic vinegar, garlic, rosemary, liquid smoke and red pepper flakes.
Pour sauce over roast and onions.
Season generously with salt and pepper.
Cover and cook on low for 8-10 hours.
Once beef is fork tender, carefully remove from slow cooker onto a cutting board and cut into pieces or pull apart with two forks.
Stir slice/shredded beef back in sauce in the slow cooker and cook for a few minutes longer.
Serve with mashed potatoes or on hoagie rolls, buns or on Texas Toast.