For the sauce, blanch the tomatoes for about 15 seconds. Remove the skin, core and dice them. Peel the onion and chop finely. Put oil in a pan and start cooking the onion until they're soft. Add the tomatoes, a teaspoon of sugar, cover and simmer about 20 minutes until the sauce has thickened. Stir in the sherry. Season the sauce with salt and pepper.
For the meatballs mix the breadcrumbs with the sherry in a bowl and let stand about 10 minutes. Toast the pine nuts in a frying pan without fat. Peel the onion and garlic and chop finely. Chop the olives. Mix the beef, pine nuts, onion, garlic, olives, crumbs, cumin, and season. Shape into small balls. Fry the meatballs.