1. Heat the oil in a large casserole over medium heat. Add the onion and cook for about 5 minutes, until soft and transparent. Stir in the zucchini, eggplant, red pepper, and garlic, and cook for 5 minutes, stirring occasionally, until the vegetables begin to soften.

2. Add the stock, tomatoes with their juices, and the chopped oregano and bring to a boil. Reduce the heat to low and partially cover the pan. Simmer about 25 minutes, stirring occasionally, until the vegetables are tender.

3. Add salt and black pepper to taste. Transfer to a serving bowl and serve immediately, garnished with oregano sprigs.