Place the oil in a medium-sized pot over medium heat. Add the onion and garlic and cook until softened, about 3 minutes. Mix in the remaining ingredients and bring to a simmer. Partially cover and very gently simmer the sauce 20 minutes, stirring occasionally. Cool the sauce to room temperature. Store in a tight-sealing jar in the refrigerator until needed. Sauce will keep for up to 2 weeks.