Slice croissants in half lengthwise; tear or cut each half into 4-6 pieces and set aside. Spray a 2 qt. baking dish with cooking spray. Arrange ½ of the croissant pieces in the bottom of the dish; top with ham, ½ of the Swiss, ½ of the Monterey Jack and chopped chives. Top the ham-and-cheese layer with remaining croissant pieces; set dish aside. In a large mixing bowl, whisk together milk, eggs, mustard, salt, black pepper, nutmeg and cayenne until well combined. Pour the egg mixture evenly over the croissants in the baking dish; cover and refrigerate for at least 1 hr. (up to 12 hrs.). Sprinkle dish with remaining cheese; bake in a preheated 350° oven, uncovered, until puffed and golden (40-45 min.). Let rest for 15 min. before serving.