Things don't really grow as such over the winter. Winter crops stand - i.e. they do their growing and then can withstand winter weather to still be edible months later. Winter cabbage, leeks, sprouts and parsnips fall in this category, so sowing now won't work.
Other things like your onions overwinter, so they get a start in the autumn, then stop and wait the winter out before starting to re-grow. Spring greens and autumn planted garlic, shallots and broad beans fall into this group. This type of crop, along with the real long standing crops that take ages to mature like purple sprouting broccoli, is what gets you over the so called 'hungry gap', when the winter crops are all eaten and the spring sown stuff is not ready yet.
Hope that makes sense
