Boil chicken breasts for approx. 45 minutes. Let cool, then pull meat off bone in thin, shredded pieces. Try to keep pieces about 1-2 inches long, as it will make it easier to work with.
Cook onion in pan until slightly translucent. Mix chicken, onion and 1 1/2 cups of cheese in large bowl. This will be your filling. Set it aside while you prepare the sauce.
Cut red peppers in half and remove seeds. Align them on a cookie sheet and place under boiler until the skin is charred black. This will require some patience, but trust me, it is worth it when it comes to removing the skin. Once the skin is burnt, remove from oven and let cool, then remove skin.
Place peppers, broth, green chilies, and crushed red pepper in blender and mix about 30 seconds. To roll the enchiladas, run one tortilla through the pepper sauce mixture until both sides are coated. Apply a thin layer of sour cream, fill with chicken filling, and roll. Place in oven-safe glass dish. Continue until all tortillas are full. You may need two dishes. Cover enchiladas with remaining sauce and sprinkle remaining cheese on top. Cover with foil and bake at 350°F for 40 min. Uncover and bake for an additional 5 minutes.