This is a very simple sauce that can be made year-round in the time it takes for a pot of pasta to cook, or whipped up quickly to use as a component in countless other recipes.
In summer, of course, you can use fresh tomatoes and a food mill (passatutto) to make your sauce, but outside of tomato season, a good passata di pomodoro (tomato puree, sometimes labeled as "strained tomatoes") is the best way to go. You can always keep a jar or two of it on hand so that you'll be able to make a quick and satisfying meal at any moment, whether it's late at night, the stores are closed, or you just don't have the time!
I like to use shallots, as one of my aunts in Italy taught me, because their flavor is a bit of a mix between onions and garlic. They are not as strong and pronounced as garlic, which might be overpowering if you are using this sauce in a more delicately flavored dish like spinach and ricotta filled cannelloni, yet they are more pungent than onions alone.
I truly believe that there's really no reason to ever use jarred pasta sauce. It usually tastes awful, is often full of completely unnecessary (and unhealthy) sugar, and when making your own sauce is so quick and easy, it doesn't even have convenience going for it.
Note: I prefer to buy passata di pomodoro in large glass jars or bottles for several reasons. It doesn't have that strange metallic taste that canned sauce can sometimes have, it won't have potentially harmful BPA leached into it from the can lining (due to tomatoes' natural acidity, they leach far more BPA from a can lining than less-acidic canned foods), and you can use just as much of it as you need and keep the rest refrigerated until you need it. Some good brands that are sold this way include Mutti, Cirio, and Bionaturae (which is also organic). They're often sold in the tomato sauce section of supermarkets, and Italian import markets will almost always carry it. Italian brands will usually have no salt added — they simply contain tomatoes — so you might need to adjust your seasoning accordingly.
In a small pot, saute 2 tablespoons olive oil and 1 small shallot over medium heat, stirring with a wooden spoon until the shallot is softened and translucent (about 3 to 4 minutes).
Add the 2 cups of tomato puree and salt to taste. Cover and adjust the heat to low. Let simmer, covered, over low heat for at least 10 minutes. If you're using it in a recipe that has you busy with other preparations, you can simmer it for 20 to 30 minutes, but even the 8 or 9 minutes it takes for your pasta to cook to al dente consistency is sufficient.
If you're using fresh basil, stir it in during the last 1 to 2 minutes of simmering.