Heat the vegetable oil in a pot over medium-high heat. Add the onions and sauté for 4-5 minutes, stirring often.
When the onions are just beginning to brown, add the remaining ingredients and stir well to combine. Simmer for 30 minutes.
Pour the sauce into a blender or food processor and purée it until it is smooth. If you are using a blender, either work in batches, filling the blender no more than a third of the way at a time with the hot sauce, or slowly add the sauce while the blender is running on low speed.
Put the sauce into a saucepan, bring to a simmer and continue to simmer, uncovered, for 1-2 hours.
The sauce will keep for several weeks in the refrigerator.