Boil enough water for 5-7 tomatoes to be able to fit in.
Slit tomatoes with an x on each one.
In batches, put 5-7 tomatoes in boiling water for a few minutes.
Pull out each one and put in an ice water bath.
Take off the skin and core. Set aside.
Heat olive oil in a pan. Add chopped onion, and cook until translucent. Add minced garlic and cook 30 seconds longer.
In a food processor, blend tomatoes, onion, and garlic (working in batches).
Simmer tomatoes, onion, garlic, and salt for 30 minutes - 1 ½ hours, depending on how you like your sauce.
Notes
I cooked mine for 1 ½ hours, making it concentrated and rich. The longer you cook the tomato sauce, the more it will cook down, and thicken. The shorter you cook it, the fresher it will taste. It is all personal preference. Taste as you go.