Cover the bottom of a pan large enough to hold all the onions in one layer with olive oil. Add the onions and roll them around so they are all covered with the oil. Add the passata di pomodoro and enough water, if needed, to just barely cover the onions. Season generously with salt and pepper (and, if using the sugar).

Bring the onions to a simmer over moderate heat, cover and then lower the heat. Let the onions braise for 20-30 minutes or so, until the onions are perfectly tender and the tomato sauce is well reduced. (The oil should have visibly separated from the tomato and the sauce should cling to the onions.) If the onions are done before the sauce has sufficiently reduced, uncover the pot and increase the flame until you've reached the consistency you want.