If it's a grey mould it could be botrytis

remove and discard as much of the affected leaves as you can. I would leave the bulbs as long as they are not affected - see of they continue to grow.
Although it's no consolation, my onions are doing very little, not helped by th rabbits who continually mow the tops off the stems.
Infact, outside is a complete disaster - I have not even been able to walk on the soil for 5 weeks now....
