Wash the chillies and remove the stalks. Wear gloves if using hot varieties. Roughly chop — there's no need for precision here.
Weigh 700g of cold water and dissolve 17.5g of non-iodised sea salt in it. Stir well until fully dissolved. If you're using tap water, leave it to stand for an hour first, this allows any chlorine to dissipate.
Pack the chopped chillies and garlic cloves into your sterilised jar. Don't pack too tightly, the brine needs to circulate around the chillies.
Pour the brine over the chillies until everything is fully submerged, leaving 3-4cm of headspace at the top.
Keeping the chillies submerged is important. This is the one step that trips people up. Fill a small ziplock bag with a little of the brine solution, seal it and press it gently on top of the chillies. It acts as a weight and seal simultaneously. A clean stone or a small Kilner jar of water also work.
Place the jar on a plate (in case it bubbles over, this is normal), cover loosely with the lid or a cloth secured with a rubber band, and leave somewhere at room temperature (18-24°C) out of direct sunlight.
Within 2-5 days you should see bubbles rising through the brine when you tip the jar gently. The brine may turn slightly cloudy. Both are excellent signs, this is your ferment working.
Every 1-2 days, open the jar briefly to "burp" it. Releasing the built-up carbon dioxide gas. Just unscrew the lid for a few seconds then replace. If using a loosely covered jar this happens naturally.
Taste from day 7. The chillies should taste pleasantly tangy and sour. If you want more sourness, leave for another 3-7 days and taste again. The fermentation timeline is entirely in your hands.
When happy with the flavour, remove the weight and strain the chillies from the brine, keeping the brine.
Blend the chillies and garlic with enough brine to reach your preferred consistency - start with half the brine and add more as needed. Blend for a full minute until completely smooth.
Taste and adjust. Add a tablespoon of apple cider vinegar if you want more sharpness. Add salt if needed. Add more brine if too thick.
Pour into sterilised glass bottles. Store in the fridge. The sauce will keep for 6 months refrigerated and the flavour continues to develop over time.
