Place all the ingredients in a crock pot. cook on low for 8 hours or on high for 4 hours
Remove the onion to a cutting board to chop into pieces
Place the beef portions in a large bowl to shred. Then add the chopped onion and 1/2 C of the broth from the crockpot and mix well.
Spray both sides of some corn tortillas with Pam spray and warm them in a hot skillet. Fill them with some of the shredded beef mixture along with a sprinkling of shredded cheese, shredded lettuce, diced tomato.