Saute the onion in the oil until soft, then add the chiles and 3 cups water. Bring to a boil, reduce heat and simmer for 30 minutes. Place the mixture in a blender and puree until smooth.
Saute the beef until browned and drain off excess fat. Add the chile sauce and beef to the beans and heat.
To assemble: Place the bean mixture on the fry bread, top with the cheese, lettuce, and tomatoes, and serve.