The francesinha is Porto's most distinctive food: a substantial layered sandwich of ham, linguiça sausage, and steak, sealed in cheese, dunked in a spiced tomato-and-beer sauce, and often accompanied by fried egg and chips. It is not a delicate thing. It was invented in the 1950s as a Portuguese adaptation of the French croque-monsieur, and it has evolved since into something with no French counterpart whatsoever.
Every restaurant in Porto serves a version; the difference between them is mainly the sauce. At the better establishments — Café Santiago on Rua de Passos Manuel and Lado B in Bonfim are consistently cited — the sauce is made fresh, balanced between heat and richness, and exactly the right consistency for dipping the bread. The mediocre versions use bottled sauce and get the ratios wrong. Order it for lunch rather than dinner: this is not food that sits lightly.