In a medium sauce pan, heat the oil on medium heat. Add the onion and garlic to the hot oil. Stir quickly as to make sure that the onion and the garlic do not brown too much. You really do not want to get color on the vegetables as you really just want to sweat them. Add kosher salt and freshly cracked black pepper to taste.
Add in the whole peeled tomatoes. Add them in and stir them around and begin to break them up in the pan. Lower the heat to medium-low and let that all simmer, stirring occasionally for about 30-35 minutes.
Check the sauce for final seasoning and add salt and pepper if needed. Check that the flavor is where you want it to be. Using either an immersion blender or a stand blender, blend the sauce until it is pureed and smooth. Check one more time for seasoning and Enjoy!