In a large skillet, combine chicken, chicken broth, and black pepper. Bring to boiling; reduce heat. Cover and simmer for 12 to 14 minutes or until chicken is no longer pink. Drain well. When cool enough to handle, use 2 forks to shred chicken into bite-size pieces. (You should have about 1 1/2 cups shredded chicken.) Set aside.
If using fresh spinach, place spinach in a steamer basket over boiling water. Reduce heat. Cover and stem for 3 to 5 minutes or until tender. (Or in a covered medium saucepan, cook fresh spinach in a small amount of boiling water for 3 to 5 minutes.) Drain well. *If using frozen or canned squeeze out water.
In a large bowl, combine chicken, spinach, and the 2 tablespoons green onion; set aside. For sauce, in a small bowl, combine sour cream, yogurt, flour, cumin, and salt. Stir in milk and chile peppers. Divide sauce in half. Spoon one portion into a storage container; cover.
For filling, combine remaining portion of the sauce and the chicken-spinach mixture. Divide the filling among the tortillas. Roll up tortillas. Place, seam sides down, in a 2-quart rectangular baking dish.
Cover baking dish with foil. Chill baking dish and sauce in storage container for at least 4 hours or up to 24 hours.
To serve, preheat oven to 350°F. Spoon sauce from storage container over tortillas. Bake casserole, covered, for 20 minutes. Uncover and bake about 20 minutes more or until heated through. Sprinkle with cheese; let stand for 5 minutes. If desired, garnish with chopped tomato or salsa, green onions, and cilantro.