Slice the onions into thin semi-circles. Put a medium sized pot over high heat and add the oil. Once the oil is hot, add the onions.
Let them cook, stirring occasionally, until they reduce by half and turn a nice brown color. It will take approximately 20 minutes, and be patient it'll make all the difference.
Once the onions are done, remove them from the heat and add the espresso, chipotle pepper, cola, ground mustard seed, pepper, soy sauce, Worcestershire and ketchup. Mix them all together. Let the sauce simmer for 20 to 30 minutes on medium heat.
(I like this sauce nice and chunky, but if you want it to be a bit smoother, feel free to blend it with a hand blender or food processor.) Pop it into an airtight container and it will keep in your fridge for a week.
Cook's Note: Be really patient and let your onions caramelize to let them get nice and rich and golden brown to add more depth to the flavor.