1. Combine brown sugar, salt, paprika, onion powder and pepper in a small bowl. Rub mixture all over brisket.
2. Warm oil in a large pot or Dutch oven over medium-high heat. Cook brisket until browned on all sides, turning with tongs, about 7 minutes total. Transfer meat to slow cooker.
3. Arrange onions on top of brisket. Whisk together cola and tomatoes in a large bowl and pour over onions and brisket. Cover and cook on low until meat is fork-tender, 7 to 8 hours.
4. Transfer meat to a cutting board and allow to stand for 10 minutes. Remove gravy from slow cooker. Slice meat against the grain and serve with gravy.