All the burgers and sandwiches come with fries. I suspect you could substitute onion rings for a nominal fee. You can also order a plate of onion rings for around $6 as either an appetizer of shared side dish.
When I was there in Sept. 08, I was able to sub the onion rings for the fries that came with the burger. I don't think it cost more than $2 extra to do so, and they were really yummy!