Coat the bottom of a skillet (use a medium, oven-safe skillet if you've got one) with oil and place over medium-low heat. Add onion and toss a few times to coat. Allow to cook, flipping occasionally, until onion is very soft and caramelized, 30-40 minutes. Add garlic. Cook until very fragrant, about 1 minute more.
The whiskey goes in next, but before adding it, check to make sure the heat is still set at medium low. If there is any sizzling in the pan, lower the heat until it subsides. Alcohol vapors are flammable, and adding whiskey to a pan that's too hot could result in ignition. Keep a lid handy and use it to put out any flames. Now slowly begin adding the whiskey. Stir it in and raise the heat to medium to bring it to a simmer. Allow to cook until most of the liquid has cooked off, about 4 minutes. Remove from heat.
I'll be totally honest here, I used 1/2 cup of maple syrup, and it was freaking delicious. I can see how that might be a bit much though, so if you object to overly sweet stuff (which I usually do myself), start with 1/4 cup and add more as needed.