To make the base layer of the fresh tomato sauce: Bring a large pot of water to a boil and set a large bowl of ice water next to it. Using a sharp knife, make an X on top of each tomato (you can core them first, if you wish, however not totally necessary as peeling the skin afterwards should take this part right off). Working in batches, drop a few tomatoes in the water at a time.
Blanch for 1-2 minutes or until you notice the skin starting to wrinkle and peel back from the flesh.
Using a slotted spoon, immediately transfer the tomatoes from the hot water to the ice bath.
Using your hands, easily peel off the skin from each tomato (you can use a knife to core out the hard tops/centers if they don't come out on their own). No need to worry about the seeds.
Place the whole tomatoes into the body of a food processor (working in batches, if you have to).
For a smooth sauce, process until completely smooth. For a chunkier sauce, pulse a couple times. Alternatively, you can use a food mill or your hands to break down the tomatoes, depending on how chunky you like your sauce.
Add 5-6 fresh basil leaves to the bottom of the mason jars you plan to use for canning.
Evenly distribute the tomato purée amongst the jars.
Seal tightly with 2 piece lids (flat metal part and screw ring) which is important for the next step.
Place the tightly sealed jars into a large pot and cover with cold water. Bring to a boil then reduce the heat to low, cover and simmer for 20-30 minutes. This will seal the jars and substantially prolong their shelf life.
The tomato sauce is ready to be stored in a cool, dry place until you're ready to use/cook it.
When ready to use/cook the sauce: Dump out the desired amount of sauce into a sauce pan and add any desired additional ingredients. Bring to a boil then reduce to a simmer for at least 30-60 minutes (longer for larger batches) in which time the sauce should thicken up nicely. Sometimes, I like to cook the veggies, onions and garlic in good quality olive oil first for extra flavour before adding the jar of tomato sauce. Use in all your favourite recipes!