Spray a 6-quart slow cooker with nonstick cooking spray.
In a medium bowl, whisk together the cream of chicken soup, sour cream, evaporated milk, Dijon mustard, Worcestershire sauce, and Montreal steak seasoning until smooth and well combined.
Add the cubed chicken and diced ham to the slow cooker.
Pour the sauce over the chicken and ham. Stir until everything is evenly coated.
Cover and cook on LOW for 4 to 5 hours, or until the chicken is tender and reaches an internal temperature of 165 degrees.
Remove the lid and stir in the mozzarella cheese until completely melted.
Add 1½ cups of the Swiss cheese and stir until the sauce is smooth and creamy.
Sprinkle the remaining ½ cup Swiss cheese over the top. Cover again for 5 to 10 minutes, or until the cheese has melted.
While the cheese is melting, stir together the crushed Ritz crackers, melted butter, Parmesan cheese, and parsley in a small bowl.
Sprinkle the buttery cracker topping over each serving just before serving to keep it crisp.
Serve over buttered egg noodles, mashed potatoes, steamed rice, or even baked potatoes.