Hot sandwiches pose a problem when packing a lunch, as it is difficult to store a sandwich intended to be served hot at the proper temperature to keep it safe. While some sandwich fillings can be held hot with the help of an insulated thermos then constructed at lunchtime, you will have to save most hot sandwiches for home to prevent spoilage.
Hot Lunch Basics
Sandwiches containing meat, fish, poultry, cooked vegetables, dressings, gravy or dairy products other than hard cheeses or yogurt must be held either below 40 degrees Fahrenheit or above 140 F to prevent spoilage and foodborne illness. While it is possible to keep your sandwich below 40 F until lunchtime, keeping it above 140 F is a much more difficult task. If you want a hot sandwich and have microwave access, use an ice pack to keep it cool and then use the microwave to heat it directly before serving. Alternatively, pack hot ingredients such as meatballs and sauce into an insulated thermos and store separately from the bread until lunchtime; construct the meatball sandwich directly before eating.