Layer onion and meat in slow cooker.
Mix chili powder, cumin, garlic powder and salt in small bowl. Sprinkle seasoning mixture all over meat. Top with green pepper. Pour diced tomatoes over top. Cover.
Cook 8 hours on LOW or 5 hours on HIGH.
Remove beef from slow cooker. Shred beef with 2 forks. Drain and discard liquid from vegetable mixture in slow cooker. Return beef and vegetables to slow cooker. Stir in cilantro. To serve, use a slotted spoon to fill warm tortillas with mixture. Serve with salsa and shredded cheese, if desired.