The easiest, fastest growing (relatively speaking!) and most productive crops you can grow in winter are salad leaves. There’s a wide choice, including:
Land cress: has a strong, assertive flavour, a little like water cress. Sow by late August / early September for winter leaves – it’s perennial so will produce leaves all year round.
Winter purslane or claytonia: a succulent winter leaf, packed with vitamin C. Attractive round leaves, that also produce pretty and very unusual white flowers in the spring. Sow late August / early September.
Pea and broad bean shoots: one of the fastest growing and easiest crops to grow in winter. Sow up to the end of October / Early Nov for Dec / Jan harvest.
Lambs lettuce: mild flavoured leaf – a good contrast to some of the stronger ones. Sow late August / early September.
Winter lettuces – some varieties are hardy enough to survive winter – arrowhead lettuce is one good variety to try. Sow in August or early September.
Rocket – good winter crop – and less prone to bolting than in the summer. Sow in late August or September.
Asian leaves including: mustard red giant, green in the snow, and mizuna. Sow in late August or early September).
Sorrel: strong, lemony leaf, lovely in salads in small quantities. Sow in August for winter leaves. This is another perennial that will produce leaves year round once established).
Other winter leaves:
Cavelo nero – tall and stately, this can look great in containers, and the leaves are actually more tender and tasty after a frost. The big leaves are best cooked, but smaller leaves can also be used in salads.
Kale – home grown kale can be tender and tasty, well worth experimenting with – I’m currently trying a heritage variety called asparagus kale.
Bright lights chard – a bright and cheerful winter crop – and one that often recovers to grow very well in the early spring.
Spinach – is less prone to bolting if grown at this time of year. Sow in August.
Coriander – coriander does surprisingly well in cold weather, I’ve had my best crops from early September sowings. It doesn’t bolt at this time of year – and although it goes nearly dormant in the coldest months,it usually comes back strong in the early spring.
Source: Vertical Veg at: http://www.verticalveg.org.uk/winter-growing-its-time-to-plan-and-sow/