In a small dry skillet, toast the cumin seeds over moderate heat, stirring constantly, until fragrant, about 2 minutes.
Grind them using a spice mill or mortar and pestle.
Heat a large enameled cast-iron casserole.
Season the beef brisket with salt and pepper.
Work in batches, add the meat to the casserole, and cook over moderately high heat until well browned all over, about 8 minutes per batch.
Transfer each batch to a large plate.
Add the minced garlic, chopped jalapenos, and onions to the casserole.
Cook over moderate heat, stirring occasionally, until softened, about 4 minutes.
Stir in the commercial chili powder (or Reno Red Chili Mix), pure red chili powder, ground coriander, and half of the ground cumin.
Cook, stirring, for 2 minutes.
Return the cooked brisket to the casserole.
Add the beef stock, chopped tomatoes with their juices, and the oregano.
Bring to a boil over moderately high heat, then lower the heat and simmer gently, stirring occasionally, for 3 hours.
Stir in the coarsely ground beef chuck.
Season with salt and continue cooking until the brisket is very tender and the sauce has thickened, about 1 hour longer.
Stir in the remaining ground cumin and simmer for 15 minutes.
Garnish with thinly sliced scallions and serve.
