After cutting off inedible tips of vegetables, wash and slice them into 1-inch squares. Sauté vegetables in half of the oil and remove them from the frying pan. Sauté onion and tomatoes in the remaining oil, and season with parsley, salt, and pepper. Put everything in a stock pot with a cup of water, cover tightly, and simmer until almost all moisture has cooked away. Serve hot, accompanied by steak, meatballs, or any other main dish.