Combine onions, garlic, cider vinegar, Worcestershire sauce, and 1/2 cup of the whiskey in a pot and cook over medium heat until the onions are translucent and soft.
In a blender, pulse ingredients until the onions are smooth and combined with the rest of the sauce. Pour back into the pot and add the remaining ingredients along with the last 1/4 cup of whiskey. Cook over low heat for 20 minutes, stirring frequently.
Use the BBQ sauce as a dip or as the sauce for pulled pork sandwiches.