1-1/2 lbs. beef chuck roast
1 medium onion, sliced
1 cup water
8 hard taco shells
8 soft flour tortillas
1 packet (1.25 oz.) taco seasoning mix
1/2 cup taco sauce
1 can (4 oz.) diced green chiles
Optional toppings: shredded lettuce, chopped tomato, shredded mild Cheddar cheese, sour cream