I read this recipe here somewhere.
Cover roast in Tony Chachere, put in slow cooker with nothing else (no liquid) on low. When you're able to shred it and put on tortillas with onions and cilantro!
I've made this several times and its awesome, it cooks fine with no added liquid and shreds perfectly. Not sure how you finish your beef but the roasts I used where from cows finished on corn.
Good luck!
