Heat the olive oil in a large saucepan over medium-high heat. Add the onions and sauté until translucent, about 6 minutes. Add the garlic and sauté until fragrant, about 1 minute. Add the tomatoes, bring the sauce to a boil, reduce the heat, and simmer for 20-25 minutes. Add the basil and simmer for 5 minutes more. Taste and season with salt and pepper.
Variation: Canned Tomato Sauce Replace the fresh tomatoes with whole canned tomatoes that have been seeded and coarsely chopped. (Reserve the juices to add to the pot as well.) Be sure to taste the tomatoes before starting the sauce. If the tomatoes need extra body or flavor, sauté 1/4-1/3 cup tomato paste with the onions after the onions are translucent. Or replace half of the whole tomatoes with tomato purée.