The Argentine pizza topped with cheese and onions and derived from focaccia is fugazza.
Fugazza originated in Buenos Aires and takes its name from fugassa, the Genoese word for focaccia. Its defining version is a thick, airy pizza crust covered with onions, often accompanied by oregano, salt and sometimes olives. Unlike many familiar pizzas, the basic fugazza does not necessarily include tomato sauce.
A closely related variation is fugazza con queso, which adds mozzarella. That cheese-topped form is often the source of confusion in trivia questions because fugazza can refer to the onion pizza generally, while fugazza con queso specifies the version with cheese. Fugazzeta is another related style, typically made with two layers of dough and cheese between them, with onions on top.
The dish is strongly associated with Buenos Aires and is commonly linked to pizza maker Juan Banchero, whose family helped popularize it. Scaccia is a Sicilian stuffed flatbread, pissaladière is a French Provençal onion tart, and lahmacun is a meat-topped flatbread, so none is the canonical answer here.