Season the beef with salt and pepper on all sides.
Heat oil in a cast iron skillet or dutch oven over medium high heat until almost smoking. When very hot, add the chuck roast and sear until deeply golden brown, about 2-3 minutes per side. Remove the chuck roast and repeat the searing process with the short ribs, searing each side until deeply golden brown. The meat will not be cooked through at this stage.
Add the chuck roast and short ribs to a slow cooker along with the onions, tomato, garlic, chiles, cinnamon, bay leaves, beef stock, cumin, oregano, and chili powder. Cover and cook on low for 8-10 hours, or on high for 6-7 hours. Stir occasionally, ensuring that the meat is covered in liquid and the chiles are submerged.
Once beef is fork-tender, remove from the slow cooker and shred using two forks. Discard any bones.
Strain and retain broth for tortilla preparation.
Add the chiles, tomato, onion, and garlic to a blender and blend until very smooth. Combine with 1 cup broth, or more for the desired consistency (for tacos, use 2 cups). Season consomé with salt to taste.
To serve as a stew, add the shredded beef to the consomé. Serve with rice
To serve as tacos, add 1 cup consomé to beef and season with salt.. Heat an oiled skillet over medium low heat.
Dip the tortilla into the broth gently, then lay it flat on a griddle or skillet. Add some of the shredded beef, cilantro, and onion (and Oaxacan cheese for quesabirria) on one side of the taco, then wait for the shell to bubble. Once you see bubbles form, fold the taco over and brush with more of the broth.
Serve tacos with extra consomé for dipping.