Preheat oven to 350 degrees. Lightly spray a 10.5" x 7.5-inch baking dish with cooking spray.
Combine chicken, ham, sour cream, Swiss cheese, dijon mustard, ranch dressing mix, and cream of chicken soup. Spread into prepared pan.
Combine crushed crackers and melted butter. Sprinkle over top of chicken mixture.
Bake uncovered 30 to 40 minutes until hot and bubbly. If the crackers start to over-brown cover with aluminum foil.
Notes
If you're not using Ritz crackers you'll need about 1 ½ cups of cracker crumbs.To make this casserole ahead of time, prepare as directed and place in baking dish. Cover tightly and store in the fridge for 2-3 days. Add the crispy topping right before baking, Then bake as directed above.Storage tips - Allow to cool completely and store leftovers in a covered container in the fridge for 3-4 days. Freeze up to three months.