How to Make It
In a medium bowl, cover all of the chiles with hot water and let stand for 30 minutes. Drain the chiles, discarding their soaking liquid.
Meanwhile, preheat the oven to 325°. Season the brisket with salt. In an ovenproof enameled cast-iron casserole, heat the oil. Sear the meat, one piece at a time, over high heat, turning once, until browned, about 10 minutes per piece; transfer the pieces to a plate as they're browned.
Add the onion, garlic, cumin and coriander to the casserole. Cook over high heat, stirring frequently, until the onion is translucent, 3 minutes. Stir in the soaked chiles, tomatoes, jalapeño and cinnamon stick. Return the brisket to the casserole. Add the stock, season the braising liquid with salt and pepper and bring to a simmer. Cover and braise in the oven until the meat is fork-tender, 2 hours; flip the meat after 1 hour.
Discard the cinnamon stick. Transfer the meat to a work surface and let cool. Using your fingers, shred the meat.
Working in batches, transfer the contents of the casserole to a blender and puree until smooth. Strain the sauce and return it to the casserole. Add the shredded meat to the sauce and season with salt and pepper. Reheat, garnish the chili with the scallions and serve.