Preheat a medium sauce pot over medium-low heat with a little oil. Add onions, salt, and sugar. Mix and cook for about 15 minutes, until onions caramelize and brown.
Raise the heat to medium and pour in whiskey. Simmer it for 2-3 minutes and then add remaining ingredients.
Lower the heat back to medium-low and cook for another 10 minutes, stirring often. (You may been to loosely cover the pot with a lid so the sauce doesn't splatter.)
Take the pot off heat to blend the sauce. You can use either an immersion blender or transfer the mixture into a standing blender or food processor. Blend well until smooth.
Storing:
Cool the sauce first to room temperature and store it in the refrigerator, in a glass jar or another food storage container with an air-tight lid. Stored properly, it should last for 7-10 days.
Notes
Disclaimer: Nutrition information shown is not guaranteed to be 100% accurate as most ingredients and brands have a slight variation.