Preheat oven to 375. Coat two 8 inch square baking pans with cooking spray.
Place chicken in large saucepan and add water to cover by 1 inch. Bring to a boil. Reduce heat and simmer until just cooked through, 10-15 minutes. Transfer to a clean cutting board, dice.
Heat oil in large skillet over medium heat. Add mushrooms and onion and cook, stirring occasionally, until all mushroom liquid has evaporated, 7-9 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 an cook, stirring, until thickened, 3 to 5 minutes. Remove from heat. Stir in tomatoes, sour cream, chiles, salt and chicken.
Spread 1 cup filling in each pan. 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 with cheese and olives.
Bake until hot and bubbly, 20 - 25 minutes. Let stand for 15 minutes before serving.
TO FREEZE - Let unbaked casserole cool to room temperature. Tightly wrap with heavy duty foil and freeze. Thaw in fridge for 2 days then bake uncovered at 375 for 50 minutes.