Francesinha is a very popular dish in Oporto, invented by Daniel David Silva, a former immigrant in France. Back to Portugal, missing the famous croque-monsieur, he came up with a big hot sandwich on a plate stuffed with sausages, beef and ham, topped by a fried egg and covered with melted cheese and spicy sauce made out of beer and tomatoes. It usually comes with french fries on the side.
In Oporto and cities nearby, people like to have it for dinner, specially if they can dance it off later, and, obviously, there are many many places all over the city specialized in francesinha and no consensus on which restaurant makes the best one, so we have to go and try some of them.
Last weekend I went to a place near the river Douro and ate the biggest francesinha I had ever seen. Although I appreciated the fact that the bread had been previously toasted, I couldn't finish it. Still, I recommend Verso em Pedra. Just don't ask for the soft sauce: instead, ask for the regular one, not too spicy.