Bring a large pot of water to a boil.
Clean and cut the tomatoes into quarters
Parboil the tomatoes for about 2-3 minutes, then drain
Using a food mill, run the tomatoes through the mill in batches. This took me about 15 minutes. The food mill will catch the skin and most of the seeds, which you can discard. (If you do not have a food mill, you can press the tomatoes against a box grater, I have not done this method myself but have heard it works well).
When you are finished you should have about 5cups of tomato puree.
Add tomato liquid to a sauce pot, with olive oil, salt, tomato paste, garlic cloves, bay leaf, and basil.
Bring to a boil and then down to a simmer. Simmer until the liquid reduces in half. This will take several hours. You don’t have to do much but keep an eye on it every once in a while.
Once it has reduced, remove the bay leaf and the basil stem, pour into a clean mason jar (or similar container) with a top that seals well.
I recommend writing the date on the jar. Sealed well, it will keep refrigerated for 1 month or frozen for 3 months.