Whilst clearing an area of the allotment I came across one of my last years onions skulking amongst the weeds. It had sprouted and on checking the Internet I found the Self sufficiency blog which talks about these onions being cooked. In fact there is a festival dedicated to them in Catalonia in Spain. You can find the link here: a self sufficient life. I will definitely try growing the sprouts further to see what happens.
My Tiny Plot also had a tip worth bearing in mind which suggests that if you have too many onion sets left over, you could plant them in pots and you will grow spring onion like onions for later in the year. I like that - waste not want not!