Sprinkle spice mixture evenly over all sides of beef, rubbing it in with your fingers.
Place beef in a shallow roasting pan or dutch oven and top with sliced onions. Pour broth and vinegar over the meat. Roast in oven, covered, for about 3 hours or until meat is fork tender.
2 medium onions, 14 ounces beef broth, 1/4 cup white wine vinegar
Let meat stand in pan juices for about 15 minutes or until it is cool enough to handle.
Remove the meat from the pan. Using a slotted spoon, remove the onions and reserve the pan juices. If you trimmed the meat well you shouldn't have too much fat on the surface, but if you do, skim off with a spoon or use a fat separator.
Halve the meat crosswise then shred it using two forks.
Stir onions into the shredded meat then add in enough of the reserved pan juices to moisten (1/2 to 1 cup). Set meat mixture aside and discard remaining pan juices.
In a medium saucepan, combine undrained chile peppers and the remaining 1/2 teaspoon chili powder; cook about one minute or until heated through. Stir in flour. Cook and stir for one minute more.
8 ounces diced green chile peppers, 1 Tablespoon all purpose flour
Remove from heat. Stir in sour cream and 1/2 cup of the cheese.
8 ounces sour cream
Increase oven temperature to 350 F. Grease or spray a 13x9 baking dish and set aside.
Divide the shredded meat mixture among tortillas, placing the meat mixture along one edge. Top with sour cream mixture and roll up tortillas (they will be full).
Arrange in baking dish, seam side down. Bake, covered, for 30 minutes.
Uncover and spoon salsa verde over the enchiladas, and sprinkle with the remaining 1 1/2 cups of cheese. Bake uncovered for 10 minutes more, or until cheese is melted and enchiladas are heated through. Enjoy!