Wash the tomatoes and remove the stalks. Peel the onions and garlic. Eighth and core the peppers. Simmer together with the curry powder in a saucepan over low heat for 25 minutes, stirring frequently.
Brush through a brisk lotte or through a sieve that is not too fine, puree with a hand blender as required.
Finely crush the chilli peppers in a mortar with a little salt. Return to the saucepan with the tomato paste, balsamic vinegar, brown sugar and the puree. Simmer for another 10 minutes while stirring. Stir the cornstarch in a little cold water until smooth and use it to thicken the ketchup.
Pour boiling hot into sterilized bottles or jars and close immediately.