Place beef and onion in slow cooker.
In small bowl combine water and seasoning mix.
Pour over beef and onion.
Cover and cook on Low for 6 to 8 hours until fork tender.
Remove beef from cooker; shred beef with two forks.
Place shredded beef in a large bowl.
Stir in sauce and chilis.
Serve with warm shells/tortillas and your favorite taco toppings.
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