Preheat oven to 425 degrees F and spray or grease a 13 x 9″ casserole.

In a medium-sized saucepan over medium-high heat, melt butter. Add flour, stir until smooth and cook for a minute. Gradually whisk in chicken broth and water. Cook over medium heat, stirring until thickened. Dip a spoon in the mixture and run your finger over the back; the line should hold a distinct shape.

Remove from heat and let cool slightly, two to three minutes, then whisk in the sour cream and green chiles. Pour about a third of the sauce in the bottom of the pan.

Combine the chicken, 8 ounces (2 cups) of the shredded cheese and 1/2 cup sliced green onion. Soften the tortillas in the microwave for about 10 seconds. Place about a half cup of the chicken mixture on each tortilla and roll, tucking in the sides as you go. Place seam side down in the casserole. Pour remaining sauce over the top.

Bake, uncovered, for 20 minutes. Sprinkle with the remaining cup or two cups cheese (however much you wish to use) and bake an additional five minutes until the cheese is melted. Sprinkle with remaining green onion before serving.

Note: this sauce is very good over any number of items, especially Chimichangas.