Take frozen chuck roast out of freezer and run under warm water, just enough to get packaging off.
Place in slow cooker with water and sliced onions.
Generously season roast.
Cook on low 6-8 hours, or until tender. Shred with fork.
Add shredded beef to gluten-free taco shells or lettuce with your favorite toppings to make into a taco salad, and enjoy!