Heat the olive oil over medium heat in a large dutch oven and add onion.
Saute the onion for 3 or 4 minutes, until translucent, and then add garlic and red pepper flakes. Keep cooking for about 20 seconds to release the flavor. It is recommended to start with fewer red pepper flakes (ie, one teaspoon) and add more at the end if desired.
Add crushed tomatoes, vinegar, salt, and black pepper.
Thin with some water ONLY if sauce is too thick. Make sure that the sauce is thick enough to coat the back of a spoon. Alternatively, you can thin it with reserved pasta water if you'll serve this sauce on pasta.
Increase heat to high, bring to slow boil (bubbles are just starting to rise to surface) , then immediately reduce heat to low, cover with lid slightly ajar, and simmer for 15 to 20 minutes.
Remove garlic if it was sliced.
Garnish with plenty of parsley or basil. You can also drizzle a bit more olive oil (2 tablespoons or so) over the sauce if desired.
This sauce is typically served with penne pasta (it Italy, often topped with pecorino Romano cheese) and can be thinned with reserved pasta water if desired.