Place chiles into a medium bowl and bring 3 cups water to a boil. Pour boiling water over chiles, cover and allow to steep for 30 minutes to soften.
Remove the chiles from the bowl and place into a blender along with a little of the liquid, reserving the rest. Puree until smooth, adding more liquid as necessary until a smooth sauce is formed. Pass through a fine mesh strainer, pressing with the back of a spoon or ladle to remove the seeds and skins; set aside.
Cut the beef into 1 - 2 inch chunks. 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 translucent, about 5 minutes. Add beef chunks, in batches if necessary, and brown evenly. Stir in chile mixture. Add beef stock to cover beef. Bring to a boil, then reduce heat to low, and simmer for 3 hours, or until meat is tender. Add more stock if necessary during cooking.
Serve with hot corn tortillas. Garnish with chopped green onion and sour cream, if desired.