Chop the zucchini, peppers and onions in the food processor. Heat the ketchup and oil in a saucepan. Put the pureed vegetables in the pot. Season with chili powder to taste.
Bring the mixture to the boil again, pour it into glasses while still hot and close. (Makes approx. 8 glasses of 0.4 liters)
I use this sauce not only for grilling, but also to season other sauces, e.g. goulash or spaghetti sauce.