Porto's cherished francesinha is one of the world's great sandwiches. You take two doorsteps of white bread, fill them with mortadella, chipolata, smoked sausages, steak, ham and cheese. After  toasting, it's wrapped in more cheese, baked, then served hot, doused in a spicy sauce that every cafe in Porto will tell you is a state secret. The line waiting outside under this unpretentious café's neon sign is a clue that many consider this the best place on the planet to eat one. The sauce's special flavor apparently comes from being made in the same pan every day for decades. You can get them with or without fries, and you can add a fried egg on top. As the name—francesinha translates as "little French girl"—the story goes that a Porto waiter returned from a spell working in France invented the sandwich in the 1950s and named it because the sauce was as piquant as his memories of Parisian mademoiselles.  Best with a cold beer.