I just let them get on with it unless we have weeks of hot, dry weather when i'll give them a soaking every few days.

We grow onions in a raised bed too. At the moment I am double digging manure into the bed. Last year we grew 6lb onions. It pays to get the soil ready at this time if year.

Make sure you keep them well watered as the top of a raised bed dries out quickly. Mine grew well once I kept them watered.

It's a plant that's supposed to be kept on the dry side....they prefer hot dry summers