Onions can be grown from seed or from sets and are either Spring or Autumn planting.  I have always grown mine from set, preferring the Autumn varieties as they encourage me out on a chilly day to clear a bed and plant them out.  I love being down at the allotment in the autumn; I take a can of soup to warm on a camping stove and a thermos of tea, get a fire going and burn off the weeds and rubbish and plant up the onion and garlic sets.  Not only do they help keep me at the 50% cultivation required but by planting them now (mid to late October), I save myself an additional job in the springtime.