in a big pot, that takes at least 3 liters, heat the oil and add the onion and the garlic.
when those are getting brown, add the tomatoes, stir and let cook for another 1-2 minutes.
Now reduce the heat to low (i turn to 3 from 9) and let it simmer for 2-3 hours without a lid.
In the beginning the brew looks almost clear with just some red sprinkles in it, but after a while it gets more red and thick.
If you wish, puree the sauce with an immersion or a normal blender, add salt to taste and fill it in clean glasses. Close them and turn them upside down until they are cooled down.
if you want a more intense basil flavor, either add more fresh basil at the end, bevor filling the sauce into the glasses or add fresh basil just before serving.