Combine 1 Tbs. brown sugar, smoked paprika, salt, cayenne, chili powder, garlic powder and black pepper. Rub the beef all over with this mixture.
Place onions and 3 tablespoons brown sugar in a 5- or 6-qt. slow cooker.
In a large skillet, heat oil over medium-high heat; brown beef, 1-2 minutes on each side.
Transfer to slow cooker; pour enchilada sauce over beef. Cook, covered, on low until beef is tender, 7-8 hours. Remove beef; shred meat with 2 forks. Return to slow cooker and heat through.