Onions are very easy and very, very good. Temp is 225-250 and around an hour and a half until tender.
I've done them whole with the outer skin peeled:
I have also cut them in half, take a little of the core out, put a half of a chicken or beef boullion cube in there and wrap in foil:
Or, since you're doing a butt, and it works with a brisket, slice up the onions and whatever other veggies you want. Put about a cup and a half of apple juice in there and place the pan below the butt to catch the drippings. Leave it on there as long as the butt is on. I did these yesterday (onions, jalapenos, and mushrooms) under a briaket: