1. Chop the tomatoes roughly and place in a pan over moderate heat.
2. Add oil, onion and salt.
3. Cook till the tomatoes are tender and oil floats to the surface.
4. Remove from heat and press the tomatoes through a strainer to remove the seeds and skin.
5. Store the puree in the fridge and use as required.
Note:- It will keep for up to a week if stored in fridge.