Preheat a saucepan over medium heat. Place the onions in the pan and saute in oil until browned (about 7 min). Add the garlic and saute for another minute. Add all the other ingredients except the mustard and liquid smoke, and cook for at least 30 minutes and up to 1 hour, uncovered, stirring occasionally.
Lower the heat if the sauce begins to splatter everywhere. Add the mustard and liquid smoke and taste for sweetness/sourness. Adjust the flavors if you think it's necessary, and cook for 5 more minutes. If you like a smooth BBQ sauce then puree it, but thats not entirely necessary.