Sol's Slow Roasted Caramelized Onions

Onions are in season at the moment and I very much wanted to taste them roasted again. The first time I had them was in Toronto, when Sol, (the sister and aunt of friends) visiting from Paris, made them.

Sol showed me how to make her roasted onions on a subsequent visit; her recipe seemed quite simple. She sprinkled a teaspoon or so each of olive oil and balsamic vinegar over each onion, followed by a sprinkling of salt and pepper. She told me she baked them for a very long time; over 12 hours.

I know I'm missing something here because after 12 hours in the oven, my onions (although good) weren't magnificent like Sol's. They also failed to caramelize throughout as hers had. I'm thinking I used too high a temperature, perhaps? I'll never know for sure, Sol passed away last year. This wonderful woman was 94.

3 lbs small vidalia or yellow onions
1/3 cup olive oil
1/3 cup balsamic vinegar
salt and freshly ground black pepper

Preheat the oven to 325ºF.

Trim off about 1/4-1/2" off the tops and bottoms of the onions and remove the skins. Lightly score a cross on the top of each onion. Place them into an oiled baking pan.

Drizzle the olive oil and vinegar over the onions and  then sprinkle them with a bit of salt and pepper.

Cover the dish with foil and bake for about 3-4 hours. I tried an onion at this point and it was cooked though and good. No point in leaving them in for an additional 8 to 9 hours as their sizes will only diminish and they won't caramelize; at least not at this temperature.

Serve hot or at room temperature.