Heat the olive oil in a large, deep skillet. Add the onion and cook over moderate heat until softened, about 5 minutes. Add the garlic and cook for 1 minute. Reduce the heat to moderately low and add the tomatoes, eggplant, zucchini, bell pepper and bay leaf. Season with salt and pepper, cover and cook, stirring occasionally, until the vegetables are very tender, about 1 hour. Discard the bay leaf and serve warm or at room temperature.