Discard the stems and some of the seeds from the chillers, then soak the chilis in water for 30 minutes, until softened
Meanwhile dry fry the coriander and cumin seeds to bring out the flavour then grind to a powder.
Pound the garlic with the salt, then add the drained chilis and pound the mixture until it is smooth
Add the spices and gradually pound in the oil, trickling it in and mixing until the sauce is well blended another mayonnaise like consistency
Use the Harrissa at once or transfer it to an airtight jar. Flood the surface with a little more olive oil may conceal. Cover tightly and store in a cool place or in the refrigerator for up to 3 weeks