Get a large pot hot. Peel and finely dice the onions. Gloss the hot pot with 1 tbsp oil. Lower the heat. Add the onion and a pinch of salt and pepper. Cook the onions for 10 mins, stirring occasionally.
Scrub the carrot. Coarsely grate 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 onion is meltingly tender, swirl in the carrot, tomato, garlic and most of the chilli. Cook over medium heat for 5 mins till the tomatoes collapse into a sticky paste.
Dissolve the stock cube in 750ml boiling water. Bring to a gentle boil. Lower heat. Simmer 5 mins. Swirl in the spinach till it wilts into the soup. Taste. Tweak seasoning to your liking. ladle into warm bowls and serve, garnished with remaining chilli.