Toast the dried chiles for 2-3 minutes in a skillet over medium heat.
Place the toasted chiles into a bowl of water for 30 minutes.
Cut the tops and seeds out of the chiles.
Season the beef with salt and pepper to taste.
Heat the oil in a skillet over medium high heat.
Sear the beef in the oil until it is browned on all sides.
Place the beef into the crockpot, and cover in the onions.
Add the chiles, vinegar, garlic, cumin,paprika, oregano, lime juice, 1 cup of the beef broth and salt and pepper to taste to a food processor. Pulse until well blended.
Pour the sauce and remaining broth into the slow cooker, and stir to combine.
Cover and cook on low for 8 hours.