Preheat the oven to 350°F. Spray a 2½-quart casserole dish with nonstick spray.
Spray a large nonstick skillet with nonstick spray. Place the pan over medium heat until hot. Add the chicken; cook 6 minutes on each side, or until done. Remove the chicken from the skillet and let cool. Shred the chicken with 2 forks; set aside.
Re-coat the skillet with nonstick spray; place over medium heat. Add the onion and garlic; sauté 5 minutes or until tender. Add the shredded chicken, beer, pepper and tomatoes; cook 15 minutes, or until most of the liquid has evaporated. Remove from heat. Reserve 1 tablespoon of the green onions and 1 tablespoon of the olives for garnish. Stir in the remaining green onions, olives and chiles into the chicken mixture; set aside.
In a medium saucepan, heat the milk over medium. In a small bowl, combine the flour, salt, cumin and coriander. Gradually whisk in the flour mixture, until blended; cook for 7 minutes or until thick, stirring constantly. Put the egg whites in a medium bowl, and very gradually whisk the hot milk mixture into the egg whites, stirring briskly until incorporated. Set aside.
Spread ½ cup of the white sauce in the bottom of the casserole dish. Arrange 4 tortilla halves over the sauce; top with 2 cups of the chicken mixture, ½ cup of the sauce and ½ cup of the cheese. Repeat the layers twice, ending with the sauce. Set aside the remaining ½ cup of the cheese.
Bake, uncovered for 40 minutes (or until hot and bubbly). Sprinkle the casserole with the remaining ½ cup of cheese, reserved green onions and olives; bake an additional 5 minutes. Let stand 10 minutes before serving. Serve garnished with sour cream and salsa.
Notes
To spice things up a little more, use Ro-Tel tomatoes.
If you aren't worried about keeping the recipe light, melt one tablespoon of butter in the pan when making the sauce- and then add the milk. And use full-fat, regular cheese and sour cream instead of light.