Season the beef generously with salt and pepper. Heat the oil in a large, deep skillet. Add the beef and cook on each side for five minutes.
Into a slow cooker, place the beef, onion, chilies, garlic, tomato paste, tomato sauce, and spices. Cover and let cook for 8 hours, or when the beef falls apart when pulled with a fork.
Remove from heat and shred the beef with two forks. Stir it back into the sauce and keep warm.