There is a quaint little cafe called Darwin's five minutes from our apartment. The place is divided into three parts: coffee counter with pastries on the right, sandwich counter on the left, and a tiny grocery department (where the husband gets his supply of beer from) in between.
So, very simply, the Leftover Sandwich is really that - made of leftovers! It only required two slices of lightly toasted bread (check out the grill marks from my pan!), spread with butter on one side and dijon mustard on the other, topped with shreds of beef that's been microwaved, one slice of large red onion, two sticks of celery heart. Nothing cast in stone - just use what you have in the fridge and hopefully the ingredients complement each other.
If I didn't tell you, would you have guessed this was made of solely leftovers?