Step 1

Heat the oil in a saute pan over medium until shimmering. Add the onions and tomatoes and stir to mix. When the tomatoes begin to bubble, reduce heat to medium-low and cover. Let cook for 30 minutes, stirring every 10 minutes.

Step 2

Uncover, add the butter, sugar and a big pinch or two of salt and continue cooking for 30 minutes, stirring often. The mixture is done when the tomato sauce has darkened (but not burned) and sticks slightly to the pan. When you pull a spatula through it, it leaves a clean trail behind it.