Heat oil in a saucepan over low heat. Add onion and garlic. Cook and stir until soft and translucent, about 5 minutes. Add passata, basil, and salt. Cover and simmer over medium heat, stirring occasionally, until tomato sauce has thickened, about 20 minutes. Remove garlic halves before serving.
Instead of the onion you can use 2 shallots. If you have the time, you can simmer the sauce for much longer for a more intense flavor.
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