if the duck is frozen, let it defrost. If it is fresh, pluck and singe the feathers and remove the innards except for the liver. Slice them into portions, wash them and then drain them well.
In a stockpot, brown the onions in water, over a medium fire. Put 1/2 cup of olive oil and sauté. Add the duck pieces and sauté them too. Douse with the vinegar and once it evaporates, add the tomatoes, the salt, the pepper, the spices, the sugar and 2 glasses of water and let it simmer for about 1 hour.
Meanwhile, sauté the quince with the remaining oil, in another pan. Then, add them to the pot and cook for another 1/2 hour. Just before removing the pan from the fire, add the lemon juice.
