Combine onion, garlic, beef broth, lime juice, tomato paste, chopped chipotle peppers and adobo sauce in slow cooker. Stir until well combined.
1 medium yellow onion, 4 cloves garlic, 1 cup low sodium beef broth, ¼ cup fresh lime juice, 3 tablespoons tomato paste, 2 chipotle peppers in adobo sauce, 1 tablespoon adobo sauce
Stir together chili powder, cumin, oregano, salt and pepper in a small bowl. Place beef chuck roast in slow cooker and sprinkle the spice mixture over the meat.
1 tablespoon chili powder, 1 tablespoon ground cumin, 2 teaspoons dried oregano, 1 teaspoon salt, ½ teaspoon black pepper, 3 pound boneless beef chuck roast
Cover and cook on low for 8-9 hours or high for 4-5 hours, until beef is very tender and shreds easily.
Transfer beef to a large bowl and let it rest for 5-10 minutes, then shred (discard any excess fat on the meat). Skim excess fat off of the cooking liquid in the slow cooker. Stir some of the cooking liquid into the shredded beef, a little at a time. Don't add all of the liquid, just enough to make the meat moist and flavorful.
Serve beef in tortillas with toppings as desired. This shredded beef also makes excellent enchiladas, taco salads and nachos.