Season the chuck roast with salt and pepper to taste.
In a skillet over medium-high heat, sear the chuck roast on all sides in some oil until browned, about 2 minutes per side. Remove and set aside.
Combine cayenne, cumin, smoked paprika and garlic with the beef broth.
Add tomato paste, jalapeno and lime juice and stir to mix.
Transfer the roast to a 6 qt. slow cooker and top with diced onions.
Pour the beef broth mixture over the roast and cook on low for 6-8 hours or high for 3-4 hours.
Shred the beef with two forks and toss to coat with the juices.
Dish and serve hot with tortillas and your favorite toppings.
