You can prepare all the separate elements well in advance, then just mix together when you're ready to serve. The chicken can be cooked 5 days in advance and stored in an airtight container in the fridge, the lettuce and spring onion can be chopped 5 days in advance and kept in a container ready for the dressing. The dressing will last for at least 1 week stored in the fridge, and the parmesan crisps and crispy prosciutto will keep well for about a week. Don't add the dressing until you're ready to serve - it will make the lettuce soggy otherwise.