Place onion in bottom of a 5-quart slow cooker; add roast and 1/2 cup water. Sprinkle 1 package taco seasoning mix over top of roast.
Cover and cook on high 5 hours. Remove roast, and shred with two forks; return to slow cooker, and stir in remaining package of taco seasoning mix. Cover and cook on high for 30 more minutes or until boiling.
Heat tortillas according to package directions. Using a slotted spoon, spoon beef mixture evenly down center of each tortilla; top with cheese and desired toppings, and roll up. Serve with Pico de Gallo.