Preheat oven to 350 degrees.
Season the Chuck Roast with salt, freshly round pepper and set aside.
Using scissors, cut off the tops of the dried guajillo and ancho chile peppers. Cut along one side of the peppers to remove seeds. Discard seeds.
Quarter onion, tomatoes and peeled garlic cloves, set aside.
Heat a large, 5 quart cast iron Dutch Oven over medium high heat. Add oil and heat until just barely beginning to smoke. Sear the chuck roast on all sides until a deep mahogany color. Reduce heat.
Add the cumin, oregano, quartered tomato, onion and cleaned chilies. Add beef stock. Bring to a boil and cover.
Place covered dish in the oven and continue to braise for 3 hours. At the end of cooking time, the Chuck Roast should be fork tender. Remove from oven and let cool in braising liquid for 30 minutes. (see note) Remove the roast from braising liquid onto a cutting board. Cover with foil to keep warm. NOTE: This can be prepared a day in advance. Let cool completely in the braising liquid. Rewarm the beef and continue with instructions. Ladle the braising liquid and solids (tomato, onions, garlic, chili peppers) into a blender. Blend until smooth. Pour into a stock pot, keep warm.
Using two forks, shred the beef into bite sized chunks. Place into a deep rimmed dish and ladle half of the sauce onto the beef.