Peel and finely dice the onions. Warm a pan over a medium heat for 1 min. Gloss the hot pan with 1 tbsp oil. Lower the heat. Add the onion and a pinch of salt and pepper. Cook the onion for 10 mins, stirring occasionally, till soft and a little caramelised.
While the onions cook, scrub the carrot. Coarsely grate or finely chop it. Finely dice the tomatoes. Peel and chop the garlic. Finely chop the chilli, discarding the seeds and white pith for less heat.
When the onions are meltingly tender, tip in 1 tsp ground coriander, the chopped carrot, tomato, garlic and most of the chilli. Cook over a medium heat for 5 mins till the tomatoes start to collapse into a sticky paste.
Dissolve the stock cube in 750ml boiling water. Pour into the pan with the veg. Bring to a gentle boil, then lower the heat. Simmer for 5 mins.
Swirl in the spinach till it wilts into the soup. Taste and tweak the seasoning to your liking. Ladle into warm bowls and serve, garnished with the remaining chopped chill