Preheat the oven to 375ºF. Grease a 9″x13″ pan and set aside.
In a skillet, brown ground beef and onions. Drain grease and return meat mixture to pan.
Stir chili powder, salt and pepper into the meat mixture.
Spoon some of the meat mixture into each tortilla and top with cheese (remember to reserve 1 cup of cheese for topping).
Roll up tortillas (I usually tuck the ends inside while rolling it) and place seam side down in your 9″×13″ pan.
In a medium size bowl, combine soup, sour cream, and chiles and pour over tortillas. (If you are making this up ahead of time, cover the pan with foil and stick it in the freezer at this point).
Bake uncovered for 20-25 minutes.
Remove from oven, sprinkle reserved cheese on top, and bake (uncovered) for an additional 10 minutes.