Heat 2 tbsp of the oil in a large skillet. Add the onions and cook over low heat for 5 minutes, until transparent, stirring occasionally.
Add the eggplant and cook for 10 more minutes.
Add the bell pepper and cook for 10 more minutes, stirring. Add the zucchini, tomatoes, and garlic, season with salt and cook for 10 more minutes. Cover and simmer gently for 1 hour.