Cut up the tomatoes into chunks, taking out any damaged bits and the hard core. Add the onion.
Put the tomatoes, onion and salt in a large saucepan on a medium heat, keeping the lid on to bring it up to boiling temperature. Removing the lid once it is boiling. You are trying to reduce and extract the water from the tomatoes
Using a tomato grinding machine, pour the mixture through, which separates the juice from the fibre of the tomatoes.
Do this three times.
Pour the juice into the shallow terracotta dishes. Lay the three wide dishes in the sun, covering with cheesecloth or netting to keep off the insects, stirring occasionally.
Bring the dishes in at night or if it rains. In August, this should take three days. Eventually the tomato juice shrinks and fills only one dish.
Scoop it into clean glass jars and top with olive oil. You can also vacuum pack the tomato concentrate.
This can be used in sauces, in stews or on toast. The stratto is so delicious I will even use it directly spooned onto hot freshly cooked pasta as a sauce in itself.