1. Layer onion and meat in slow cooker.
2. 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.
3. Cook 8 hours on LOW or 5 hours on HIGH.
4. Remove beef from slow cooker. Shred beef with 2 forks. Drain liquid from remaining vegetable mixture. Return beef and vegetables to slow cooker. Stir in cilantro. Mix and heat with sauce. To serve, use a slotted spoon to fill warm tortillas with mixture.
Test Kitchen Tip: For extra heat, use spicy salsa and/or add 1 chopped jalapeno pepper to slow cooker when adding green pepper
Slow Cooker Tip: For best results, do not remove cover during cooking.