Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In 10-inch skillet, melt butter over medium-high heat. Add mushrooms; cook 4 to 6 minutes, stirring frequently, until mushrooms are browned.
In large bowl, mix soup, milk, mustard, salt, thyme and pepper. Stir in mushrooms, chicken, broccoli and water chestnuts. Pour into baking dish. Sprinkle with cheese.
Cover dish with foil. Bake 30 minutes. Sprinkle French-fried onions over top. Bake uncovered about 15 minutes longer or until mixture is hot and bubbly.