Crop rotation

All the books say follow brassicas with roots and onions. Problem is my brassica bed wont be empty in time to plant my carrots, shallots onions etc, what is a girl to do. How do I get around that.
this veg growing is great, but really confusing sometimes

I wouldn't get too stressed about rotation. As long as you move everything round each year strict rotation is not crucial on an amateur level, but obviously the soil prep from the previous crops may suit some crops better than others.