Pissaladière is a classic dish shared by Genoa, Italy and Nice, France. The two cities sit close to the same border. Similar regional ingredients can be found used in the local foods of each region. It is usually served using a dough similar to pizza. In Provence, you can also find puff pastry as the base as Julia Child used. I have used both and sometimes serve it in a tart shell of pâte brisée.
Although not traditional, I have spread a layer of Gruyère (because I love Gruyère) as a first layer over the base before adding the onions. I’ll leave that up to you.