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Add the olive oil to a large pan over medium heat. Add the onion and saute until soft but not browned, 5 to 6 minutes. Add the garlic and the red pepper if using and cook for another 2 to 3 minutes.
Add the pureed tomatoes and season with salt and pepper. Reduce the heat to medium-low and simmer for about 30 minutes. Strive for balance in the consistency of the sauce; it should be fluid but shouldn't look overly wet. A few minutes before taking the sauce off the heat, add half the basil and season with salt and pepper. Add the remaining basil just before serving.