Rinse the dried New Mexico chiles and place them in a pot with 3 cups of water. Bring to a boil, then remove from heat and let steep for 30 minutes until soft, using a plate to keep them submerged if needed.

While chiles soak, trim and cut the beef chuck roast into bite-sized chunks. Mix flour, salt, and pepper in a bowl, then dredge each beef piece in the seasoned flour.

Heat olive oil in a pan and brown the beef in batches. Transfer the browned beef to a slow cooker layered with chopped onions.

Strain the soaked chiles, reserving the liquid. Blend the softened chiles with enough reserved liquid to create a smooth sauce, then strain through a fine-mesh sieve. Pour the red chile sauce over the beef and onions in the slow cooker.

Add beef stock to just cover the meat. Cook on low for 8-10 hours or high for 4-5 hours until the beef is tender. Adjust seasoning with chili powder if desired.