Since onion sizes and amount you need can vary greatly I won't be very specific for most ingredient amounts needed. Core onions but important: do not core all the way through. You want the "bottom"
Add a dollop of base or maybe a tablespoon or so of bouillon into onion with some minced garlic and rosemary, top off with olive oil. Place in pan and slow roast at 225 degrees for 3-4 hours.
Smoke with your preferred wood. I like cherry, apple or hickory for most vegetables. Some of the mix will spill into the pan, do not discard this. When done remove outer skin which will be tough.
Cut up onions into the same pan they were cooked in. You want all those juices. Use the onions as a side or on your favorite sandwiches. This goes perfect with my pulled pork recipe on here too!