Remove the stems from the chillies. If you don’t want the sauce to be too hot, also remove the seeds.
Dry-roast the coriander and cumin seeds until they start popping and they release their aroma.
Grind the spices into a powder. This can be done by hand, with a mortar and pestle. Or with the help of a spice grinder, coffee grinder or blender.
Put the chillies, garlic and salt into a blender. Process until finely chopped.
Add the ground spices to the blender, together with some of the olive oil. You might not need to add all the olive oil. Only add enough to reach a mayonnaise-like consistency.
Stored in the fridge, your homemade harissa will keep for about a week.