For many, growing vegetables means planting in spring and harvesting during the summer and early autumn. The supermarket supplies for the ten months of the year when the vegetable patch is unproductive. It doesn't have to be like this. with careful planning, or even some casual research, you can be eating you home grown for most of the year. Of course a polytunnel or greenhouse would help, but its not essential.
Cabbages, carrots, swedes, brussel sprouts, potatoes, leeks, turnips and radishes can all be harvested right into the winter. You can plant chard, cabbages, beets, kale, leeks and onions in the autumn for early spring crops. These lists aren't anywhere near exhaustive, just for starters.
With a some wire and a bit of fleece you can keep growing, and harvesting, greens all through the year.
Do a little research on storing vegetables. many keep better in the ground than out: leeks can keep well until late January. Other can be frozen, often after blanching. And you don't have to be French to plait your onions. Man has been devising ways to store vegetables for almost as long as its been growing them.