There are various types of perpetual onions or 'bunching onions' which will continue to crop year after year, especially if protected from the worst of the elements. I have some onions called 'Cuisse de Poulet' which grow in the polytunnel which are still going after two years.
The only thing you need to do is make sure the plants are fed from time to time. These onions are particularly useful in the middle of winter when there is nothing else growing.