Puree the tomatoes until smooth in a food processor.
Add the olive oil to a large saucepan over medium heat. When hot, add the onions and saute just until soft, but before they actually start to brown. Add the garlic. If you'd like to make the sauce spicier, add the pinch of red pepper. Add the pureed tomatoes and season with salt and pepper.
Simmer over a medium-low flame 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 fire, stir in the basil and season with salt and pepper.