Heat the oil in a large saucepan over low heat, add the onions, and cook for 10 to 15 minutes, until the onions are translucent but not browned. Add the garlic and cook for 1 more minute. Add the tomato paste, vinegar, honey, Worcestershire sauce, mustard, soy sauce, hoisin sauce, chili powder, cumin, and red pepper flakes. Bring to a boil, then lower the heat and simmer uncovered for 30 minutes. Use the sauce immediately or pour into a container and refrigerate.