Step 1

Preheat oven to 375 degrees F. Coat two 8-inch-square baking dishes with cooking spray.

Step 2

Place chicken in a large saucepan and add water to cover by 1 inch. Bring to a boil. Reduce heat and simmer until just cooked through, 10 to 15 minutes. Transfer to a clean cutting board. Dice when cool.

Step 3

Meanwhile, heat oil in a large skillet over medium heat. Add mushrooms and onion and cook, stirring occasionally, until all the mushroom liquid has evaporated, 7 to 9 minutes. Add bell pepper and cook, stirring, until just tender, about 3 minutes. Add garlic and chili powder; stir for 1 minute. Add flour and cook, stirring, for 30 seconds. Add broth and cook, stirring, until thickened, 3 to 5 minutes. Remove from heat. Stir in tomatoes, sour cream, chiles, salt and the chicken.

Step 4

To assemble: Spread 1 cup filling in each prepared baking dish. Layer on 4 tortilla halves and top with 1 cup filling. Repeat 2 more layers of tortillas and filling, ending with the rest of the filling. Sprinkle cheese and olives on top.

Step 5

To serve: Bake until hot and bubbly, 20 to 25 minutes. Let stand for 15 minutes before serving. To freeze: Let unbaked casserole(s) cool to room temperature. Tightly wrap with heavy-duty foil (or freezer paper) and freeze. (To prevent foil from sticking to the cheese, coat with cooking spray first.)