Place chiles and 3 cups water into a medium stockpot, and bring to a boil. Remove from heat and steep for 30 minutes to soften. Strain into a bowl, reserving the cooking liquid. Place the chiles and some of the liquid into a blender, and puree until smooth. Add more liquid as necessary to form a smooth sauce. Pass sauce through a fine mesh strainer to remove any seeds and the tough skins; set aside.
Cut the roast into 1 to 2 inch chunks. In a medium bowl, combine flour, salt, and pepper. Dredge the beef chunks in the seasoned flour; set aside.
Heat olive oil in a large pot over medium heat. Saute onion until tender and translucent, about 5 minutes. Add beef chunks a few at a time, so as not to overcrowd the pot, and cook until evenly brown. Remove cooked meat, and continue browning remaining meat. Return reserved cooked meat to the pot. Stir in pureed chile mixture. Add beef stock to just cover beef chunks, or to personal preference. Bring to a boil over medium heat. Reduce heat to lowest setting, and simmer for 3 hours, or until meat is tender. If necessary, adjust with more stock during cooking.
Serve with chopped onion, sliced green onion, shredded cheddar cheese, and sour cream.
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