From Southern Living - The flavor of this casserole is incredible. Very mild heat. My pre-teen kids love these even with the spinach. Delicious. I hope you all enjoy as much as my family has over the years.
Place chicken in Dutch oven; add salt and water to cover. Bring to a boil; cover, reduce heat, and simmer 40 minutes or until tender. Remove chicken, let cool slightly. Dice or shred chicken, set aside.
Cook spinach according to package directions; drain well, and reserve 1 cup spinach broth. Set aside.
Saute onion in butter in Dutch Oven until tender. Stir in spinach and chicken, set aside.
Combine soup, reserved spinach broth, milk, green chiles, sour cream, and half of cheese in bowl; mix well. Stir half of sauce mixture into chicken mixture. Reserve remaining sauce.
Spoon chicken mixture evenly down center of each tortilla; roll up tortillas, and place seam side down, in 2 lightly greased 13 X 9 X 2 inch backing dishes. Spoon remaining sauce over tortillas. Bake uncovered at 350 degrees for 25 minutes (I cook mine about 45 minutes to an hour). Sprinkle with additional cheese, back an additional 5 minutes.