Add the sliced onion to the bottom of a slow cooker.
Season the beef with the fajita seasoning on all sides and place on top of the sliced onion.
Top the roast with the ranch seasoning, pickled jalapeño with some of the juice, and the optional butter.
Cover and cook on low for 8-10 hours until the beef easily shreds with a fork.
Transfer the beef to a bowl and shred before adding back to the remaining juices along with some fresh cilantro and lime wedges (if you plan to make tacos or Tex-Mex dishes). Set the slow cooker to its keep warm function and let the beef rest for 30-60 minutes before serving, if possible.
Notes
*Chuck roast works great as a replacement. Leaner options include eye of round roast (115 calories, 23 grams of protein, and 2 grams of fat per 100 grams raw) and bottom round roast (140 calories, 22 grams of protein, and 5 grams of fat per 100 grams raw). Compare that to the 175 calories, 19 grams of protein, and 11 grams of fat in 100 grams of raw beef shoulder roast. Source: USDA Food Database.**I used H-E-B beef fajita seasoning, which is a blend of salt, spices, and a meat tenderizer. Similar products: Bolner's Fiesta, Meat Church Dia de La Fajita Rub, or McCormick fajita rub with tenderizer. You can also add natural tenderizers like pineapple or kiwi. Read more about tenderizing with kiwi and enzymes in my grilled flanken short ribs recipe.