To a large cast-iron skillet over medium-high heat, add oil.
Once the oil is hot, add the chuck roast to the skillet and sear on each side, just enough to brown (3-5 minutes per side).
Add the seared beef to a crockpot. Top with Carroll Shelby’s Birria Taco Seasoning and the enchilada sauce.
Cover and slow cook on low for 10 hours, or until the meat is tender and easily pulls apart.
When ready, remove the chuck roast from the slow cooker and use two forks to shred. Add the shredded roast back to the juices in the slow cooker.
Preheat a skillet to medium-high heat.
Butter one side of each slice of bread.
Working one or two at a time, add bread, buttered side down into the skillet. While it is cooking, top with ½ cup shredded beef, about 1 tablespoon of white onions, ¼ cup queso fresco, ¼ cup mozzarella, cilantro, and another slice of bread, buttered side up.
Cook over medium-low heat for 5-7 minutes; then, flip over the sandwich and cook another 5-7 minutes, or until golden brown. Use a spatula to remove from the skillet.
Serve with leftover juices from the slow cooker for dipping.