Rinse chuck roast under cold water and pat dry with paper towels.
Season roast with salt and pepper.
Heat 2 tablespoons butter in a large cast iron skillet over medium heat.
Once butter has melted, add seasoned roast to the skillet. Cook until golden brown on both sides, about 5 minutes per side.
Place the seared roast in the bottom of a slow cooker.
Add remaining 2 tablespoons butter to the skillet. Add diced onions and cook them until soft and translucent, about 4-5 minutes.
Add garlic, salt, cayenne, chili powder, cumin, pepper and chopped cilantro.
Stir to coat the onions with the minced garlic and spices.
Add beef broth and diced green chiles to the pan and stir to combine. Bring to a simmer.
Pour sauce over roast in the slow cooker. Cover and cook on low heat for 8 hours.
Remove roast from the slow cooker. Shred beef and return shredded beef to the slow cooker.
Melt a tablespoon butter in a large skillet. Dip a corn tortilla in the sauce and place wet tortilla in the skillet. Cook tortilla on first side until soft, about 30-45 seconds.
Flip tortilla over and top with a tablespoon shredded Jack cheese, a few slices of pickled Jalapeño slices, and about a ¼ cup of the shredded beef.
Fold tortilla in half enclosing beef and cheese filling. Fry until cheese melts, filling is heated through and corn tortillas are nice and crispy.
Serve with a dollop of sour cream if desired.