As you have figured out Zebo, Onions do not do so well in sunless areas and short of putting up a tunnel or making a lot of effort for little return, you may be better having a rethink. You could source a mirror system if you can direct sunlight onto the patch but you would be tasked with turning and securing the things. Salad onions may do best but if you are determined to put onions sets in you could try Bedfordshire Champion, Red Arrow, or Santero F1. Ask your supplier for what is known as a short day variety, which are strangely enough readily available in the States but not here. Other vegetables take readily to shady areas including. Mange Tout Peas, Beetroot Boltardy, Salad Onions, Summer Cauliflower, Kohl Rabi, Brussels Sprout Bosworth, Early Potatoes, Dwarf French Beans, Cabbage, Radish.