Summarize the following:
Combine the chopped onions, olive oil, garlic, cumin, oregano, chiles, cider vinegar, and salt in a blender or food processor. Blend the ingredients together at medium to high speed until a thick puree forms. Do not drain the adobo sauce from the chipotle chiles before adding them. You should pour the sauce into the blender along with the chiles. Spread the sauce into the bottom of your slow cooker. Place the beef on top of it, and pour the beef broth over the meat. Place the bay leaf on top.  Coat the beef in the sauce by turning it a few times, dipping all sides in the sauce at least once in the process. Use a 5-qt to 7-qt (5-L to 7-L) slow cooker for best results. Cut the beef into several chunks, if necessary, to fit it into the slow cooker. If you want it done sooner, set the slow cooker to high and cook the beef for 6 hours. Resist the temptation to open the lid and peek at your meal as it cooks. Doing so releases the trapped steam, and that steam is partially responsible for cooking the meat. As a result, the overall cooking time can increase 15 to 30 minutes every time you open the slow cooker. Remove the beef from the slow cooker and place it on a cutting board or rimmed baking sheet. Use two forks to shred the beef into pieces, working from the outside in.  While shredding the beef, you should also discard any fat. A rimmed baking sheet or cutting board works best since some of the juices inside the beef can run as you work with it. Without a rimmed surface, these juices can create a big mess. Transfer the shredded beef to a large bowl. Ladle a little of the cooking liquid into the bowl, mixing after each addition. Add enough sauce to keep the beef thoroughly moist, but do not add so much that the beef drown in it or becomes soupy. If you have some cooking liquid left behind after you coat the beef, you can fill a spare ice cube tray with it and freeze the liquid for later use. These cubes of barbacoa sauce can be used to flavor stocks, soups, gravies, and other dishes later on. If you are not ready to serve the barbacoa yet, return it to the slow cooker and switch the appliance to its "warm" setting. You can keep it warm using this method for another 2 to 4 hours. After that point, you should consider transferring any uneaten beef to airtight containers and storing it in the refrigerator. Reheat it in the microwave after refrigerator storage. The beef barbacoa is ready to eat. Serve it to your family or guests while still warm, and offer it with an accompaniment of tortillas, onion, cilantro, and salsa.

Summary:
Puree the sauce ingredients. Layer the sauce, beef, and broth in the slow cooker. Cover and cook on low for 8 to 10 hours. Shred the beef. Mix the shredded beef with the barbacoa sauce. Keep it warm in the slow cooker. Serve.