Heat olive oil in a wide shallow pan over medium heat.
Add onion or shallot and cook until starting to soften, don't let it brown.
Add chopped tomatoes and salt.
Simmer, uncovered, for about 30 minutes, adding water as needed if it starts to stick. Don't cook any longer than one hour.
If desired, process briefly in a blender, or with an immersion blender, to get a slightly smoother sauce.