No, it's not a rude joke about an onion, it's this year's new disease. I've been gardening this plot for ten years now, and yet there seems to be a new pest or disease every year. Last year it was leek moth - oh, and codling moth - and this year it's a new one: onion smut (I think). Both my red onions and my shallots seem to have it. The tops go soft and have a black sooty deposit on them, and then the base of the bulbs develops a white soggy mould.
If I've made the correct diagnosis, this means that I can't grow onions or shallots on that ground for 5 years (or 8 years?). Has anyone else suffered with this? What did I do wrong?