In a five litre pot, sweat onions, carrots and celery with olive oil and salt until soft, and onion appears translucent. This will take 3 – 5 minutes.

Add chopped garlic, pepper, cumin, coriander, chilli flakes, lemongrass, kaffir lime leaves and sugar and continue to cook, stirring regularly until colour deepens.

Add tinned tomatoes, water and beans and bring to a boil.

Skim off any scum that has risen to the top, and lower heat to a simmer. Cook for a further 45 minutes before adding sweet potatoes.

Cook for another 15 minutes or until sweet potatoes are cooked through.

Optional: To thicken soup, puree a third of it and stir into the remaining soup.

Serve with lots of chopped coriander, spring onion, lime wedges and a generous drizzle of extra virgin olive oil.