These are best made a day ahead of time. Make the meat and onions, store in separate containers in the fridge, and assemble and broil to serve.
To save uneaten sandwiches, wrap in parchment paper, then place in a ziplock bag. Refrigerate for up to three days. Unwrap the sandwiches, place on a baking sheet and place in a 300F oven for 10-15 minutes or until warmed through.