Peel the cucumbers if you like, then slice them very thinly. You can do this with a sharp knife, but I prefer to use an old mandolin to get really fine, even slices. Place the cucumbers into a large bowl.
Add the shallot or red onion along with the caraway seeds, black mustard seeds and salt. Tumble everything together well, then leave the mixture to sit for 20–30 minutes. The salt will draw plenty of liquid from the cucumbers, creating a natural brine.
Pack the cucumbers tightly into a clean 1-litre Kilner jar or similar, pressing them down firmly so they are completely submerged in the brine.
Fill a small food-safe plastic bag with water and place it over the cucumbers to keep them submerged beneath the surface of the brine. This is important, as anything exposed to the air is more likely to spoil.
Sit the lid on the jar without clipping it tightly shut so the ferment can breathe and any gases can escape.
Leave the jar in a cool place for 1–3 weeks. Check it every few days and have a taste as it develops. Once the cucumbers have reached a tangy flavour you like, seal the jar and transfer it to the fridge.
They're ready to eat straight away and are wonderful alongside grilled steak, shellfish, good cheese, fatty charcuterie, smoked fish or anything cooked over the fire.
