Season the beef chuck roast generously on all sides with kosher salt, black pepper, and garlic powder.
Heat vegetable oil in a large cast-iron skillet over medium-high heat until shimmering.
Sear the roast for 3–4 minutes per side until a deep, dark brown crust forms.
Transfer the seared roast into the slow cooker insert.
Pour the beef broth, pickled jalapeño slices, and jalapeño juice over and around the roast.
Cover and cook on LOW for 7 to 8 hours until the beef is fork-tender and pulls apart easily.
During the last 15 minutes of cooking, scatter cheese cubes over the meat if desired, allowing them to soften in the steam.
Shred the meat using two tongs or forks directly in the juices before serving warm.