Heat the oil in a medium saucepan over medium heat, and sauté the onions and garlic for 2-3 minutes or until the onions wilt.
Turn off the heat, then add the rest of the ingredients: ketchup, apple cider vinegar, chili powder, honey, Worcestershire sauce, smoked paprika, molasses, liquid smoke, red pepper flakes, and celery seeds. Stir well until fully combined.
Taste and adjust seasonings to preference with more spices, more honey for sweetness, or more vinegar for more tang.
Once you're happy with the flavor, you can use it immediately, or cover it and let it cool completely. Then pour it into a glass jar or an airtight container and refrigerate to allow the flavors to meld.