Cooking up onions, without oil
Aaronut's answer to a related question outlined the differences between non-oil methods of cooking onions (such as steaming or sweating). The taste will be different because different chemical reactions are occurring.

If you want to sauté onions without a lot of oil, try oil sprayers (Pam, or hand-pump sprayers loaded with your favourite oil). This will minimise the amount of oil you use. Using less oil means you have to move them around more often to prevent burning. (If you fry onions by themselves, they will probably just dehydrate and burn.)