Heat the oil in a medium saucepan over medium heat.

Add the onion and garlic and allow them to sweat, stirring frequently, for about 10 minutes, or until translucent.

Add the tomato purée and one-half of the basil and bring the mixture to a simmer.

Immediately lower the heat and cook it, at a bare simmer, stirring frequently, for about 40 minutes, or until 90 percent of the moisture has evaporated.

Stir in the remaining basil and season with salt and pepper to taste.

Remove the pan from the heat and allow it to cool.

Store the mixture, covered and refrigerated, for up to 5 days, or freeze it, tightly covered and labeled, for up to 3 months.