I've always known it to be lots of butter and oil, caramelised onions as you say, then a frying to brown the potatoes, but then things are returned to the pan, covered and popped into the oven to sort of coalesce. This renders them beautifully soft and unctuous with - as you say - the sweetness of the onion and lots of seasoning, of course. However, I would not call them 'fried' in a strict sense.
https://www.cuisineaz.com/recettes/p...ise-28631.aspx