Coat the bottoms of a cookie sheet with your olive oil. (Use a cookie sheet with sides)
Lay the seasoned slices of onion uniformly onto the cookie sheet.
Pour the balsamic vinegar evenly over the top surface of the onion slices.
Place the cookie sheet in the refrigerator for 1/2 hour, then flip the onions over and put back in the refrigerator for another 1/2 hour to let the onions marinate.
Preheat your pellet smoker to 300°F.
Remove the cookie sheet from the refrigerator and place it onto your grill. Let cook for about an hour.
Check the onions every 20 minutes to make sure the sugars from the balsamic vinegar don’t burn.
If you notice the vinegar starting to burn, reduce the heat of your grill.
The onions are done when they are mostly transparent, but still have a little bit of white color left in the middle.
Pull the onions off the grill and serve immediately.