1. In a 3-quart saucepan place chicken in enough water to cover. Bring to boiling; reduce heat. Cover and simmer about 15 minutes or until chicken is no longer pink. Remove chicken from saucepan. When cool enough to handle, use a fork to shred chicken into bite-size pieces. (You should have about 1-1/2 cups.) Set aside.
2. If using fresh spinach, place spinach in a steamer basket over boiling water. Reduce heat. Steam, covered, for 3 to 5 minutes or until tender. (Or, cook in a small amount of boiling water, covered, for 3 to 5 minutes.) Drain well.
3. In a large bowl combine chicken, spinach, and green onions; set aside. In a bowl combine sour cream, yogurt, flour, cumin, and salt. Stir in milk and chili peppers. Divide sauce in half. Set one portion aside.
4. For filling, combine one portion of the sauce and the chicken-spinach mixture. Divide the filling among the tortillas. Roll up tortillas. Place, seam side down, in an ungreased 2-quart rectangular baking dish.
5. Spoon reserved portion of sauce over tortillas. Bake, uncovered, in a 350 degree F oven about 25 minutes or until heated through. Sprinkle with cheese; let stand for 5 minutes. Transfer to a serving platter. To serve, if desired, garnish with chopped tomato or salsa and additional green onion. Makes 6 enchiladas.