In a large skillet, heat a generous glug of oil over medium heat. Saute the onion, stirring occasionally, until soft and starting to turn golden. Add the garlic and cook for another minute, then add the eggplant and cook for another 5 minutes, adding more oil if needed, until the eggplant is soft. Add the pepper and zucchini, season with salt and pepper and cook for about 10 minutes, until everything is nice and soft and you're starting to get some golden edges. Add the tomatoes, tomato paste and oregano and cook for 5 more minutes.
Taste and adjust the seasoning if you need to; if you're adding fresh spinach or basil, stir it in and let it wilt. Serve immediately or refrigerate overnight and reheat as you need it.