Preparation / Directions:
In a large, heavy-bottomed saucepan, sauté the onion in the oil over medium-high heat until golden brown, about 7 to 10 minutes.
Add all the remaining ingredients, bring to a boil, then reduce the heat and simmer uncovered at the lowest possible heat for 4 hours. (This long cooking removes as much acidity as possible from the tomatoes.)
Puree sauce in 2, 3, or more batches to prevent it from spilling out of your food processor or blender.
Will keep 2 weeks, covered, in the refrigerator.