Heat the oil and cook the onion
and garlic until soft and golden.
Add the kidney beans and their liquid and stir in the tomato paste
and cook for a few minutes.
Add the lime juice to the kidney bean mixture. Take the pan off the heat and when it has cooled a little, process the mixture until it is a bumpy puree. When it is cool, arrange in small bowls and sprinkle over the reserved lime zest