Using gloves, remove the stem and seeds from the chilies. Scissors work really well for this, oftentimes better than knives.
Heat the chilies in a heavy saucepan over medium until fragrant, about 3 to 5 minutes. Add the beef stock and heat to a simmer. Cover and let steep 30 minutes.
Drain, reserving the peppers and the stock. Add the peppers and tomatoes to a food processor or blender, and purée. Use a little of the stock if needed.
Heat that saucepan up again, this time with about 2 tbs oil. Working in batches as needed, brown the meat, then transfer to the slow cooker.
Sprinkle in the cumin, oregano, cloves, cinnamon, thyme, and bay leaves.
Cook the onions long enough to get some color, about 5 minutes, then toss in the garlic to cook another minute. While the onions are cooking, go ahead and add the purée to the slow cooker, over the meat.
Pour in the remaining stock and use it to scrape up any browned bits.
Cover and cook on low until the meat is very tender, about 6 to 8 hours. By the end, you'll be able to shred it with two forks.
Remove the cinnamon stick and bay leaves. Serve it up with tortillas and plenty of sides like sliced radishes, cilantro, and onion. It's wonderful to eat with a spoon, or by filling up a tortilla with the meat and dipping it in the broth.