Peel the onion from the husk, finely chop, fry until golden brown. Cut the peeled tomatoes into small cubes, add to the onion, add salt and pepper to taste, simmer over medium heat under a lid for 15-20 minutes.
Tomatoes will become quite soft. Transfer the resulting tomato puree into a saucepan, add 1 liter of water, bring to a boil.