Heat the olive oil in a large skillet over medium-high heat. Add the cubed chuck roast and sear for 2-3 minutes per side until browned.
Transfer the seared beef to a slow cooker. Sprinkle the taco seasoning mix evenly over the beef.
Add the beef broth, diced tomatoes, chopped onion, jalapeño, and minced garlic to the slow cooker. Stir gently to combine.
Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the beef is tender and easily shreds with a fork.
Once cooked, shred the beef directly in the slow cooker using two forks. Stir the shredded beef into the sauce to coat evenly.
Warm the flour tortillas in a dry skillet or microwave until soft and pliable.
Assemble the tacos by spooning the beef mixture onto the center of each tortilla. Top with shredded cheddar cheese, shredded lettuce, sour cream, and chopped cilantro.
Serve immediately and enjoy!
Notes
Sear the beef for extra flavor. Let slow cooker do the work while you prepare toppings.