1. Ask the butcher to cut the liver and separate the 'dark' part from the 'white' part. Peel the onions, cut them in half, and then slice them. Pour the olive oil into a saucepan, and when it heats up, add the onions and sauté for about 25 - 30 minutes until they caramelize. Stir occasionally. Remove the onions from the saucepan and place them in a sieve. Return any oil that drains out back into the saucepan. Add the 'white' liver, which you have rinsed well (do not drain completely, and add a little more water; this will help it cook), and fry for 10 minutes.
2. Next, add the 'dark' liver, which you also rinse and drain slightly, and continue frying for another 10 minutes. Stir occasionally. Pour in the Commandaria wine and boil for 10 minutes so the alcohol evaporates and the liver softens. Add the onions back and stir for 2 - 3 minutes. Add the tomato paste, rubbing it on the bottom of the saucepan to release its aromas. Add the grated tomatoes, the sugar, and simmer the dish over low heat for another 15 minutes for the flavors to meld. Stir occasionally.
3. Before taking the dish off the heat, add the salt (at the end so the liver doesn't toughen) and the pepper, and stir for 4 - 5 minutes. The liver tastes even better when reheated.
