From Marcella Hazen's Essentials of Classic Italian Cooking
1 28-oz can of San Marzano canned whole tomatoes
5 Tablespoons unsalted butter
1 medium yellow onion, peeled and cut in half
Combine the tomatoes, their juices, the butter, and the onion halves in a medium saucepan. Place over medium heat and bring to a simmer. Cook, uncovered, at a very slow but steady simmer, adjusting the heat as necessary, for about 45 minutes, or until droplets of fat float free from the tomato. Stir occasionally, mashing any large pieces of tomato with the back of a wooden spoon. Discard the onion before tossing the sauce with pasta. This recipe will be enough for 1 pound of pasta.
The ingredients --this is it!!
put everything in a pot and let simmer
for 45 minutes until sauce thickens (reduces a bit)
see? --thicker sauce after simmering for 45 minutes uncovered
discard the onion to your compost bin