Scorch the chillies with a blowtorch until the skin blisters and blackens, then peel and de seed.
Toast the coriander and caraway seeds in a dry pan until fragrant. Place in a pestle and mortar and grind to a fine powder.
Place the chillies, garlic cloves, vinegar and tomato puree in a blender and process to a paste.
Add olive oil and spices and blend again.
«For a more mild harissa, replace half the chillies with a large red pepper.»