Heat the olive oil in a saucepan over a medium-low heat, add most of the onions (reserving a little to finish the soup) and saute for about 5 minutes, or until softened. Add the garlic, chillies and cumin and stir for a minute.
Add the stock, roasted tomato sauce or passata, tomatoes, beans, oregano and sugar. Season with salt and pepper, bring to the boil and simmer gently for 10 minutes.
Remove from the heat and add the lime juice and coriander. Taste and adjust the seasoning if necessary.
Serve the soup topped with dollops of soured cream, if you like, and scattered with the reserved red onion, chopped coriander and freshly ground pepper.
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