Peel 2 onions and chop finely. Knead minced meat, egg, diced onion and breadcrumbs well. Season with salt and pepper. Form small balls from the dough. Heat 4 tablespoons of olive oil in a pan and fry the balls in it until golden brown.
Peel the remaining onion and garlic and dice finely. Heat the remaining oil in a pot and add the diced onion and garlic. Add tomatoes with juice, tomato paste, wine and sherry.
Season with salt, pepper and chilli powder and simmer for 15 minutes. Dice the olives and add. Arrange meatballs and tomato sauce in small bowls.