Marcella Hazan's Tomato Sauce

• 1 (28-ounce) can peeled plum tomatoes, no salt or herbs added
• 5 tablespoons unsalted butter
• 1 small white onion, peeled and cut in half
• Kosher salt

Put a 3-quart saucepan over medium-high heat.
Add the tomatoes, butter, onion halves, and a pinch of salt.
Bring to a simmer, and then lower the heat.
Crush the tomatoes lightly with the back of a spoon as they cook, and stir occasionally.
Simmer very gently for 45 minutes, or until droplets of fat appear on the surface of the tomatoes.
Remove and discard the onion.