Cut the onions in half, then slice very thinly. You can use a good, sharp knife or a mandoline. Add the onions to a large bowl and sprinkle the sugar and salt over them. Stir well to combine and leave for 2 hours to soften and release all the liquid goodness.

After 2 hours, to a large saucepan, add the lard, oil or butter over low heat. Once melted, add the onions and liquid.

Cover with a lid and cook over low heat for 2 hours, stirring occasionally.

Add the tomatoes, stir well, cover partially and cook for another 1 hour. Remember that you want it thick and without much liquid. If you still have liquid in the pan, cook further until evaporated with the lid off.

Serve on crusty bread.