While the skillet is warming, pat your chicken as dry as possible using kitchen roll/paper towel.
If using garlic, add to the skillet after the onions
The finished product should yield moist chicken swimming in a mixture of its own juices plus those of the onions.
You can take it one step further and empty the softened caramelized onions, along with the juices and garlic into a blender. Blend on high till you see the mixture turn into a thick creamy sauce. Season to taste, or leave as is