Clean all the vegetables very well.
Peel the carrots and remove ends, sad or discolored leaves and parts.
Cut the zucchini, celery, carrots, fennel, potatoes and onion (if using) into small chunks, more or less the same size.
Slice the leafy greens, cabbage and leeks (if using).
Mix all the vegetables together.
Heat the oil in a large pot over medium heat. It may seem like a lot of oil but this is what makes the soup really delicious.
Add the cut mixed vegetables, salt and pepper, mix well and saute for about 10 minutes, stirring every minute or two so all the vegetables get a chance to brown ever so slightly on the bottom of the pan.
Add water (or stock if you prefer) just to cover the vegetables and cook over low heat for 15-25 minutes, just until all the vegetables are fork tender (cooking time will vary depending on how finely your vegetables are cut). If you see the soup is too thick, just add a bit of water. Season with salt and add the apple cider vinegar.