Start by washing the tomatoes. Cut any large ones into wedges and leave the small ones whole, then sterilise your jar.
Make a brine of just over 2.5 percent by whisking the salt into tepid water until it dissolves completely. Pack the jar with the green tomatoes, tucking in chillies, basil, garlic and peppercorns as you go.
Pour the brine over the top. It’s important that everything stays submerged, so you’ll need to weigh the tomatoes down. You can fill a small plastic bag with water, knot it and rest it on top. I prefer a stretchy silicone lid, placed upside down in the neck of the jar and filled with any leftover brine or fresh water. Then fit the jar lid.
Over the first few days the brine will turn cloudy, which is perfectly normal. A thin, flat white film may also form on the surface. This is harmless kahm yeast, and you can simply skim it off with a clean spoon. If you see anything fuzzy, or blue, green, black or pink, or it smells unpleasant, discard the batch.
Leave the tomatoes to ferment in a cool, dark place, such as a larder, for 8 to 10 days. Taste them along the way, and once they’re tangy enough for you, move them to the fridge. Chilling gives them a lovely, gentle fizz. They’ll keep for up to 4 months.