In a large saucepan saute the onion in the oil over medium-high heat until golden brown. (7-10 min). Add all remaining ingredients , bring to a boil, then reduce heat and simmer uncovered at 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 spilling out of blender/food processor.