Cut the onion in half, slice it thinly.
Heat a little oil in a large, heavy-based saucepan and fry the onion gently until soft.
Peel the sweet potatoes, cut them into cubes, then add them to the pan, together with the thyme leaves and the spices.
Pour in the tinned tomatoes and fish soup, add the drained chickpeas. Bring to the boil, then simmer for 20-25 minutes.
Cut the fish fillets into 3-4 cm pieces and add these to the soup, then cook for 5 minutes under cover.
Meanwhile, grate the lemon, chop the parsley. Put these in a mortar, add the minced garlic, and make a rough paste with the pestle.
Taste the soup, season to taste with salt and pepper, then stir in the paste and squeeze in the juice of the lemon
Serve with wholesome, crusty bread or flatbread.