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Peel and coarsely chop the onions and garlic cloves. Rinse the tomatoes, remove the green and hard parts and chop the tomatoes into chunks.
Heat a bit of olive oil in a large pan and sauté the coarsely chopped onion and garlic for a few minutes. Add the tomato chunks along with a small layer of water to prevent them from burning.
Let them simmer for about half an hour, the exact time depends of the tomatoes you're using. Tomatoes with lots of pulp need more time to get to the desired thickness.
You'll get the best result if you're using a food mill, this way the passata will be super smooth. You can also use an immersion or regular blender, but it won't be smooth because of the tomato skins. Season with pepper and salt.
You can use this passata for all kinds of basic sauces, such as pasta or pizza sauce.