Add the chiles and garlic to a saucepan, cover with water and bring to a boil. Remove from the heat and steep for 20 to 30 minutes. Pour off half the cooking liquid. Using an immersion blender, puree the peppers, garlic and remaining liquid until smooth. (Alternatively, you can use a regular blender. Be careful when blending hot or warm ingredients.)
Preheat the oven to 275 degrees F.
Heat the oil in a Dutch oven over medium-high heat. Season the chuck roast with the salt, pepper, chile powder and cumin. When the oil is hot, add the meat to the Dutch oven and brown on all sides, about 3 minutes, then remove to a plate.
Add the tomato paste and onion to the pot and cook, stirring, for 1 minute. Add the stock, sugar, bay leaves and pepper puree to the pot and stir to combine. Return the beef to the pot and cover. Roast until the meat is fall-apart tender, 2 1/2 to 3 hours.
Remove the bay leaves. Shred the meat in the pot and serve on warm tortillas with shaved cabbage, sour cream, lime and cilantro.