Trim beef of any excess fat. Place in the slow cooker. Slice half the onion lengthwise into strips and add to the slow cooker with the beef.
In a blender, add diced tomatoes, garlic, the other half of the onion, lime juice, cilantro, crushed red pepper flakes, cumin, oregano, chili powder, cayenne, and salt and pepper. Pulse to blend a few times until consistency is smooth and everything is incorporated. Pour the tomato mixture over the beef and onions.
Set slow cooker to low and cook for 7 hours.
Turn on your broiler. Once cooking is done, shred the beef with two forks. Using a slotted spoon, transfer beef to an oven proof dish, and broil the meat for 5 minutes to get it crispy. While beef is broiling, toast up corn tortillas and prepare your garnishes.
Remove beef from the broiler, serve with tortillas and garnishes and enjoy!
Recipe by Total Noms at http://totalnoms.com/2013/09/slow-cooker-shredded-beef-tacos/