Heat the oil in a large sauce pan (cast iron seems to be best). Cook the tomatoes, onions, and seasonings over med-hi heat for 20 minutes or so, stirring frequently. You can puree part or all the sauce if you want it smoother, or leave it lumpy and just break up the tomatoes with a spoon. The Fennil tends to reduce the acid of the tomatoes and adds a nice flavor, but you can also use just a pinch of cinnamon, or a little sugar. Experiment with the seasonings to get just the taste you like.