Friggione - Bologna Onion Soup

Although most of the visit to my blog happen during the working week, I think that many people like me love to read in the Sunday morning, and sometimes they also try to find some new idea for the forthcoming lunch.
I'll try then, starting this week, to present "Sunday recipes", simple but full of tradition and taste, and as the first one a recipe that is what more "bolognese" you can find, even more than tortellini, that were copied by the cuisines of both Modena and Romagna (with its cappelletti), the "friggione".
It's a poor and rural dish, a recipe apparently simple that needs care and some little tricks to be successful and that could be used both as a side dish or a starter, served with toasted bread.

Ingredients for approximately 4 persons: 4 2/5 oz. golden onions, 5 tablespoons of Passata (tomato puree), 8 tablespoons of olive oil, salt, water, aromatic vinegar.

Preparation: slice the onions along their longest side and put them in a large pan together with the oil and a little water. Sear the onions at very very very (did I say very?) low heat for at least two hours, adding some water every now and then because they shouldn't overcook. This is a trick that is usually missing from most recipes but in my experience I never found onions watery enough that didn't need some water added, unless you cook them on a cigarette lighter of course!
When onions will become of an uniform pinkish color add salt as you like and the tomato puree, raise then the heat so to condense the food.
Serve friggione as a side dish or with toasted croutones adding in this case some dashes of aromatic vinegar.