Pour the beans through a sieve, catching the liquid. Puree the beans with half of the liquid. Peel the onions and garlic. Finely chop the onions. Press the garlic through. Heat the olive oil and sauté the onions and garlic until translucent. Add the bean puree and sauté over a low heat for about 5 minutes.
Season the sauce with salt, pepper and chili powder.