Concentrated Tomato Sauce (Salsa di Concentrato di Pomodoro)

Heat the oil in a large heavy nonreactive saucepan. Add the onion and cook over moderate heat, stirring often, until golden. Stir in the tomato paste, sugar and a pinch each of salt and pepper. Reduce the heat to low and cook for 1 hour, stirring often, until golden. Stir in the tomato paste, sugar and a pinch each of salt and pepper. Reduce the heat to low and cook for 1 hour, stirring often; the oil will remain partially separate throughout cooking.