Heat vegetable oil in a saucepan over medium heat, then cook the onions and garlic until the onions are translucent (approximately 5 minutes).
Add the remaining ingredients to the saucepan, stirring often, and bring the mix to a quick boil, then reduce the heat.
Let the sauce simmer uncovered for 20 to 25 minutes, stirring approximately every 5 minutes. The sauce will thicken during this time. Remove from heat when it has reached the thickness you prefer.
Pour the sauce into a glass bowl and allow it to cool for 15 to 20 minutes prior to use.