Cut a x on bottom of each tomato (to be able to peel).
Put into boiling water, just enough to fit, usually 4 - 6 at a time. Once water is boiling again, cook for about 8 seconds. Remove. Cool.
Hold over a bowl and squish a little bit to get some liquid out of the tomatoes. No need to remove all, but some tomatoes have more water than others.
Chop roughly, put in slow cooker.
Add onion, salt, pepper and basil.
Cook for 6 hours on low in slow cooker or 2 to 4 hours in a pot on stove. The important thing is to reduce a lot of watery liquid from the tomatoes.
Taste sauce and adjust for sweetness and salt and pepper.
Blend with immersion blender or blender.
You may choose to add in some butter and stir to make a very smooth sauce after you have blended it.