try any of the winter brassicas - cabbage, winter sprouting broccolis, caulis, sprouts.. though you need to check pH and may need to adjust with lime.
I put some leeks after the first earlies last year, and I have only just finished them, rather yummy.
You could also look into sowing a green manure (rather than keeping it bare) - some even are suitable for overwintering. Buckwheat for example, although not suitable for overwintering, would grow very fast, choking out weeds and produce a lot of organic matter. After that, you could plant overwintering onions/broad beans...