Put the tomatoes with juices, halved onion, garlic cloves, olive oil, oregano, red pepper flakes and salt into a medium, heavy-bottomed saucepan,
Bring the sauce to a simmer over medium-high heat. Lower the heat to keep the sauce at a slow, steady simmer. Stir occasionally, and after 15 minutes or so, use a strong wooden spoon to crush the tomatoes against the side of the pan. Continue simmering for a further 30 minutes, or until droplets of oil float free of the tomatoes.
Remove the pan from the heat and discard the onion. Crush the garlic cloves and smaller pieces of onion against the side of the pan with a fork. Use the wooden spoon to crush the tomatoes. If you want a smoother sauce, use a blender.