Outdoors, sow in modules, in a seedbed for transplanting, or in situ where they are to crop:
(To possibly cover with cloches or frames later in autumn.)
Beetroot, brocoletto 'Cima di Rapa', early 'Nantes' type carrots for late autumn cropping, cabbages (red round head**, 'Greyhound' and leafy non-hearting spring types), peas (for pea shoots), sugar loaf and leaf chicory*, radicchios*, endives, Japanese overwintering onions**, salad onions, Claytonia (winter purslane/miner's lettuce)**, lambs lettuce**, American landcress**, winter lettuces, kales, radishes, rocket, Swiss chard and leaf beets*, summer spinach, summer turnips, Chinese cabbage* and other oriental greens such as Choy Sum, Pak choi, mibuna, mizuna, mustards 'Red & Green Frills', Chinese kale (Kailaan), Komatsuna**, winter radishes, quick maturing salad mixes, parsley, chervil*, buckler-leaved and French sorrel. Sow fast growing green manures like buckwheat, red clover, mustard (a brassica so careful with rotations) and Phacelia, to improve soil, 'lock-up' carbon and feed worms (digging them in later after the first frosts, then covering to protect soil, preventing nutrient loss and possible pollution), on any empty patches of ground cleared of crops that won't be used over winter.,
(*Early Aug. only, ** mid-late Aug.)
If you don't get some things sown now they won't have enough time to develop to crop well over winter, as with the shortening days now all growth slows dramatically in a couple of weeks. And another thing! - Remember these are just suggestions - you don't have to sow them all! If I was forced to choose only 5 things to grow over the winter in my polytunnel they would be Ragged Jack Kale, ruby or silver Swiss chards, watercress, lettuce Lattughino and sugar loaf chicory. They are all incredibly productive all winter in a polytunnel.
N.B. Sow in the evenings if possible as germination of some seeds can sometimes be affected or even prevented altogether by too high a temperature during the first 24 - 48 hours - this applies particularly to lettuce, spinach and also greenhouse sown carrots. Protect module-sown seedlings outside from heavy rain or strong sunlight with a plastic mesh such as 'Enviromesh' - which also protects against carrot root fly, cabbage white caterpillars and cabbage root fly - all of which can still decimate unprotected seedlings. Old net curtains work well too! Sowing in modules on a table or raised area outside also provides seedlings with good protection from slugs.