Combine the tomatoes (and their juices), halved onion, garlic cloves, olive oil, oregano, and red pepper flakes in a medium, heavy-bottomed saucepan (if using).

Over medium-high heat, bring the sauce to a simmer, then reduce the heat to maintain a slow, constant simmer for 45 minutes, or until oil floats clear of the tomatoes. After around 15 minutes, stir occasionally and smash the tomatoes against the side of the pot with a heavy wooden spoon.

Remove the onion from the pot and turn off the heat. Use a fork to pound the garlic cloves against the pot's side, and then whisk the crushed garlic into the sauce. With any microscopic onion pieces you may find, repeat the process. Crush the tomatoes to your preferences with the wooden spoon (you can blend this sauce smooth with an immersion blender or stand blender, if desired).

Add salt as desired (the tomatoes are already pretty salty, so you might just need a pinch). Serve hot. For up to 4 days, this sauce keeps nicely when covered and kept in the fridge. Keep it frozen for up to six months.