King Ranch Chicken Casserole

Sauce:
2 tablespoons unsalted butter
2 garlic cloves
1/4 teaspoon quality chili powder
Pinch of ground cumin
2 tablespoons all-purpose flour
3/4 cup unsalted chicken stock
1/2 cup milk
2 tablespoons sour cream
salt and pepper to taste
Filling:
1 tablespoon unsalted butter
1/2 medium green bell pepper, chopped
1/2 medium onion, chopped
1/4 cup fresh green New Mexican or poblano chile, chopped
8 ounces fresh mushrooms, chopped fine
1/2 cup canned diced tomatoes
2 tablespoons pimientos
Vegetable oil for frying
8 corn tortillas
3 cups shredded cooked chicken, diced
1 1/2 cups grated mild cheddar cheese
1/4 cup sliced scallions
1/4 cup sliced pimiento-stuffed green olives
Directions
Preheat oven to 350 degrees F. Grease a 9" x 12" baking dish.
In a medium skillet, melt the butter. Add the garlic, chili powder, and cumin, and saute for a couple of minutes. Sprinkle in the flour, stirring. Slowly pour the chicken stock and milk into the skillet, stirring continuously to avoid lumps. Simmer the sauce for about 3 minutes, or until it has thickened.
Stir in the sour cream, salt, and pepper. Reserve the sauce for later. (The sauce can also be made ahead and refrigerated.)
To make the filling, melt the butter in a skillet. Add all of the rest of the filling ingredients, and saute until softened and combined well. Reserve the filling.
Heat about 1/2 inch of the oil in a small skillet. With a pair of tongs, dunk each tortilla into the oil for a few seconds, or just long enough to soften them.
In the baking dish, layer half the tortillas and chicken a third of the sauce, and one half each of the filling, the cheese, scallions, and olives. Spoon over another third of the sauce, then repeat the layering of the remaining ingredients. End with the last of the sauce.
Bake the King Ranch chicken casserole for 30 minutes, just until it is heated throughout, and bubbly.