Combine the onions, celery and carrot with ½ teaspoon of salt and 2 tablespoons of olive oil in a sauce pan and sauté over medium high until the vegetables are soft and the onion is translucent.
Add the tomatoes to the pot and simmer for 30-45 minutes. Adjust the heat so the simmer is steady but gentle. Stir occasionally.
For a smooth consistency, optionally use an immersion blender to puree the sauce.
Before serving, add salt to taste.