In a small bowl combine the kosher salt, black pepper, chili powder, smoked paprika, garlic powder, dried oregano, cumin, coriander, and cayenne until the spices are evenly distributed.
Pat the beef chuck roast dry with paper towels. Brush a coating of the dijon mustard all over the chuck roast. Then evenly sprinkle the spice mixture all over until the beef is well coated. Place it into a slow cooker along with ¼ cup of the bbq sauce and water, Mix until combined. Cover the slow cooker with the lid and cook for 5-6 hours on high or 10-12 hours on low. The beef should be tender and easy to pull apart.
Remove the beef from the slow cooker and shred it. Reserve ½ a cup and 2 tablespoons of the cooking liquid that's inside the slow cooker. Discard the rest of the cooking liquid. Place the shredded beef back into the slow cooker along with the remaining ½ cup and 2 tablespoons of bbq sauce. Cook on high for another 30 minutes.
Prepare the buns to your liking. Add a mound of the shredded bbq beef to each bottom bun. Top them with a slice of pepper jack cheese. To melt the cheese, place the open-faced sandwiches onto a baking sheet and under the broiler for about 2 minutes until melted.
Remove the sandwiches from under the broiler and top with pickled jalapeños and the top bun. Enjoy!