Procedure:
Chop the onions roughly and brown in 8 tablespoons of olive oil in a saucepan. Meanwhile carefully pit both green and black olives, cut into reasonably-sized pieces and add to the onions when these have turned golden.
Cook gently over medium heat for about a quarter-hour, stirring with a wooden spoon, so that the olives are fully blended with the onions. Then add the tomato purée or canned tomatoes. Continue cooking over medium heat, stirring frequently, and season with salt and lots of chilli pepper. Simmer for about an hour.