It was very topsy-turvy last season as far as our onions were concerned with the autumn planted sets not only out performing those planted in the spring but also keeping better.
Autumn set are planted straight into the ground. As with much of our planting we plant through weed control fabric. Channels are cut into the fabric and the onion sets are planted along them and either a well rotted manure or wood chipping mulch (whichever is available is applied). I always found weeding between onions to be a problem. It was difficult to avoid bending their leaves or uprooting bulbs in the process. The weed control fabric makes this easier and also avoids weeds out-competing the onions.
At the beginning of July we started pulling the autumn onions as we needed them and in mid August we lifted all those remaining.
Autumn onions are not supposed to store well but these onions kept well and we were still using them after we had pulled the spring planted ones.