I had a sandwich in Madrid which I've recreated often since, which changed how I view sandwiches.
Bacon, cheese and egg it was. There was me, all festival-ed out, and I needed a sarnie. After it arrived, I knew I was in for a good time. The yolk of the egg was poking through a circular hole cut in the top slice of bread - which means that a) you have a little circle of bread to dip in your yolk and b) the egg yolk doesn't burst against the "flat" slice of bread, and you end up with a much neater sandwich.
I recommend making one of these.