Trim beef to remove fat; cut meat into large cubes about 2" x 2" (save fat & meat trimmings for latter).
Coat the bottom of a large crock-pot cooker with a little olive or canola oil to prevent sticking, then place meat in the cooker with the onion, Cajun spice, and garlic.
Mix together remaining ingredients (sauce) and pour over meat.
Cover and cook on high for 4 to 5 hours, or until meat shreds easily - stir once or twice during cooking if you can.
Remove the meat & onion from cooker with a slotted spoon and shred the meat with a potato masher or 2 forks, then pour about 1/2 cup of the sauce over the meat - serve with the remaining sauce on the side for those people that would like more sauce (or just save the extra sauce for other uses latter).
If you not afraid of a few extra calories, boil the trimmed off meat & fat with a cup of water in the microwave (or on the stove) and then add just the "liquid" from that to the crock-pot. I like to trim the fat off the meat because not everyone likes fat in their sandwich, but the fat is full of flavor :).