In a medium-sized, heavy-bottomed pot, pour in the tomatoes along with their juices. Add the halved onion, whole garlic cloves, olive oil, dried oregano, and red pepper flakes if using.

Bring the mixture to a simmer over medium-high heat. Once it begins to bubble, reduce the heat to maintain a gentle simmer. Allow it to cook for about 45 minutes, stirring occasionally. After 15 minutes, use a wooden spoon to gently crush the tomatoes against the side of the pot.

Once the sauce is ready, take it off the heat. Discard the onion halves. Smash the garlic cloves against the pot's side using a fork, then stir the mashed garlic back into the sauce. You can also crush any small onion bits you might find. Adjust the texture of the sauce to your preference by further breaking down the tomatoes, or use an immersion blender if you want a smoother consistency.

Taste the sauce and add salt if needed. Serve warm over pasta or use as desired. This sauce can be refrigerated for up to 4 days or frozen for up to 6 months.