Lots of good organic matter, be it compost or soil improver...

A good source of potash, such as seaweed or K-Mag, helps. If your soil is heavy clay, add sand and/or lime or gypsum--depending on the pH--along with the organic matter. Onions also benefit immensely from mycorrhizae, especially Glomus species. If you like mild, sweet onions, avoid fertilizers and soil conditioners with sulfur in them--such as ammonium sulfate or gypsum. If you like strong onions that keep well, pour on the sulfur!