Place the chopped tomatoes, tomato puree, chopped garlic, and half a teaspoon of salt and half a teaspoon of pepper in a food processor and blitz for 30 seconds.
Heat the olive oil over a medium heat and add the finely chopped onion. Saute for 3 - 4 minutes until it starts to brown and soften. Be careful that it doesn't burn.
Add the vinegar, it will sizzle and then stir before adding the chopped tomato mixture.
Bring the mixture to a gentle simmer, add the chopped basil and cook uncovered for 10 - 15 minutes, stirring occasionally, until the sauce thickens.
Season with a little more salt and pepper if required.
Notes
This recipe should make approx 2.5 - 3 cups of marinara sauce.