Heat oven to 375°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In 12-inch skillet, melt butter over medium-high heat. Cook mushrooms in butter 6 to 8 minutes, stirring occasionally, until tender. Meanwhile, cook green beans as directed on package; drain.
In large bowl, mix mushrooms, chicken, soup, water chestnuts, sour cream, pepper, garlic powder and green beans. Gently stir in roasted peppers. Spoon into baking dish.
Place 20 of the crackers in resealable food-storage plastic bag; seal bag and crush with rolling pin. Sprinkle crushed crackers over chicken mixture.
Bake uncovered 15 minutes. Sprinkle cheese over casserole. Insert remaining 16 crackers so they are sticking up to look like tombstones. Bake 10 to 15 minutes longer or until bubbly and cheese is melted.