by Cook's Illustrated
1. Hear the oil in a large, heavy-bottomed saucepan over medium heat until hot and shimmering (but not smoking). Add the onion and saute until golden brown, 7 to 10 minutes, stirrring frequently. Add the remaining ingredients. Bring to a boil, then reduce the heat to the lowest possible setting and simmer, uncovered, until thckened, 2 to 2.5 hours.
2. Puress the sauce, in batches if necessary, in a blender or the workbowl of a food processor. Transfer to a bowl and use immediately or cover in an airtight container.