Coat slow cooker bowl with nonstick cooking spray. Whisk together evaporated milk, marinade, mustard, salt and pepper; set aside.
Add potatoes, chicken and onion to slow cooker bowl and stir to combine. Pour marinade mixture over top. Cover and cook on HIGH for 2-1/2 hours or LOW for 4-1/2 hours, stirring once halfway through.
Remove cover and stir. Sprinkle with cheese and parsley and replace cover for 30 minutes or until cheese has melted; serve immediately.