Trim your brisket if it has a lot of fat. Season well with salt and pepper. Both sides.
Add the brisket to a very hot pan. Brown on both sides. My slow cooker has a browning feature, so I can do it directly in my slow cooker. The lazy girl in me loves that I do not have to dirty an extra pan.
Transfer the brisket to your slow cooker. Cover the top with sliced onions.
Mix together tomato paste, barbecue sauce, Worcestershire sauce, Dijon, garlic, and brown sugar. Season with salt and pepper.
Pour the sauce over the brisket.
Pour over root beer.
Cover and cook on LOW for 6 hours.
Slice and serve.