Coat the chuck roast on all sides with the chili powder, kosher salt, and black pepper, then place it in the slow cooker. The seasoning should cover the surface evenly before the wet ingredients are added.

Pour in the barbecue sauce and Worcestershire sauce, then scatter in the onion, garlic, brown sugar, jalapeños, and half of the crumbled bacon. The roast should sit beneath a thick, fragrant layer of sauce and aromatics.

Cover the slow cooker and cook for 6 to 8 hours on LOW or 4 to 6 hours on HIGH. The beef is ready when a fork slides in easily and the meat separates with very little resistance.

Transfer the roast to a suitable work surface and pull it apart with two forks, removing any large pieces of fat or connective tissue. Look for tender strands rather than firm chunks, with a darkened glaze and a rich barbecue aroma.

Return the shredded beef to the cooking juices, then remove some liquid if the mixture looks too loose for sandwiches and season with salt and pepper as needed. The finished beef should be moist and coated, not swimming in liquid.

Build each sandwich with sliced cheese on the bun first, followed by warm shredded beef, extra bacon and jalapeños if desired, and crispy fried onions. Serve while the cheese is soft and the fried onions are still crisp.