In a medium pot over medium, add the olive oil, onion, garlic, and red pepper flakes. Season with salt, and cook, stirring occasionally, until onions are translucent and softened.

Add the can of whole peeled plum tomatoes with its juices, crushing the tomatoes with your hands. Season with salt, dried oregano, and sugar if using.

Bring the sauce to a simmer, reduce heat to medium low, and cook 30 minutes uncovered.

Remove from heat and use an immersion blender to puree on low (or cool to room temperature and puree in a blender or food processor). Stir in the basil leaves and serve!