Heat a medium-sized pot with the oil on medium heat. When the oil is not, but not smoking, add the onions/shallots.
Sweat until fragrant and translucent, stirring occasionally Add crushed tomatoes and stir and bring to a boil. Once it comes to a boil, reduce the temperature to a simmer. Place the lid on the pot leaving a little gap.
Reduce the sauce until it has thickened to a sauce consistency, stirring occasionally. Add salt to taste and the basil torn into shreds.
You can make this is in large batches and preserve for future use.