One option is to give the tomatoes a roof, for example with lightweight tomato greenhouses or protective covers. Then the unripe tomatoes can stay outside for longer and still have a chance to use the late summer weather for ripening. However, they should not be exposed to frost.
Another option is to harvest the unripe tomatoes and allow them to ripen off indoors. To do this, wrap them in newspaper and place them on top of a cupboard or in another warm place at approx. 18 – 20 degrees Celsius. After a few days, the tomatoes should have turned red. You can speed up this ripening process by adding an apple.
If the tomato plant is still bearing a large number of green tomatoes, you can also cut off the whole plant at the stem and hang it upside down in a warm room. The room does not have to be bright, a warm boiler room is therefore also suitable. However, remove the leaves before hanging up.
You can either eat the now ripened tomatoes straight away or cut them into slices and allow them to dry. To preserve the tomatoes, place them in oil. Dried tomatoes taste even better if you place them in oil with various Mediterranean herbs.
You should not eat green/unriped tomatoes as they contain tomatine, which is also hardly diminished when heated.