Make Ahead
Onion mixture can be made up to 2 days in advance. Cover and refrigerate until ready to use.
Creative Leftovers
These sandwiches taste even better the following day! Tightly wrap unsliced sandwich with plastic wrap; refrigerate overnight. Unwrap, slice and serve.
Variation Use Your Favourite Deli Meat
Use any deli meat you like. Try salami, roast beef or smoked turkey. Or, try a combination of meats.