In a cast iron skillet, heat a small amount of olive oil. Add onions and saute until translucent. Add all the remaining vegetables, season with salt and pepper and bring to a simmer. Stir well and cover the pan. Let cook on a low flame for about 20 minutes, stirring often. Remove cover and continue to cook vegetables until tender and no cooking liquid remains in the skillet, about 15 minutes more. Adjust seasoning and serve hot, or cool down.