In a large storage bag, mix the skirt steak, garlic powder or minced garlic, cumin, salt, Italian seasonings, black pepper, and onion powder.
Seal the bag and marinate the steak in the refrigerator overnight.
Thinly slice the onions. A spiralizer can be used for uniform slices.
Place the sliced onions in the bottom of a large slow cooker.
Lay the marinated skirt steak on top of the onions. Add ½ cup of water.
Cover the slow cooker and cook on low for 8 hours.
Remove the steak from the slow cooker and shred or pull it apart using two forks.
Serve the shredded steak on a platter with the cooked onions. Optionally, serve with gluten-free tortillas and your favorite toppings for a delicious steak taco.
