Peel the onions and cut them into rings. Cut the mushrooms into slices. Cut the peppers into narrow strips. Cut the tomatoes into slices. Turn the schnitzel in flour, then egg, then breadcrumbs and fry in a pan with fat. Keep the fried schnitzel warm in the oven.
Briefly fry the onions in the fat, then add the mushrooms and season with salt and pepper. Fry the peppers briefly, then add the tomato slices. Fry everything briefly. Fill the pan with a little water, but only enough so that the vegetables are not covered. Season with broth and simmer for 10-15 minutes. Finally add the ketchup and season to taste. Use water sparingly, so you don`t need to thicken the sauce.