Place onions and 3 tablespoons brown sugar in a 5- or 6-qt. slow cooker. Combine remaining brown sugar and the next 6 ingredients; coat beef with mixture., 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, 7-9 hours. Remove beef; shred meat with 2 forks. Return to slow cooker; heat through. If desired, serve over potatoes.