Transfer the tomatoes to a large pot and add salt.
Raise the heat to medium and cook for 10 minutes until the tomatoes are soft.
With a ladle, scoop some tomatoes into a potato ricer and press to release the juice.
Open the ricer and discard the skin and seeds that have collected. Repeat this process with all of the tomatoes.
Pour the mixture through a fine mesh strainer to catch any seeds or peel that remain.
Pour the tomato mixture into a pot and raise the heat to medium.
Cook uncovered for 15 minutes so that any excess water can evaporate.
Now put the basil into a jar (or split it between 3 jars) and pour the passata on top. The passata is ready to use as is or as a base for a soup or pasta sauce.