Add the diced onion and minced garlic to the bottom of the slow cooker.
Place the brisket pieces on top of the onion and garlic.
In a small bowl, stir together the barbecue sauce, brown sugar, Dijon mustard, salt, and black pepper until smooth.
Pour the sauce mixture over the brisket.
Add the drained pinto beans and stir gently to coat everything evenly.
Cover and cook on low for 7 hours until the brisket is very tender.
Stir once more before serving to distribute the sauce.