Put the oil in a large pot on the stove to heat. Cut the onion into large cubes and put it to fry in a separate heated pot with a little oil with basil. Once you've fried the onions, remove them from the fire.
Add the tomatoes in the container with the oil and leave to boil over high heat. Prior to removing from the heat, add the onion and salt to taste. Leave it to cool. It's then ready for consumption.
Enjoy your meal!