With basic ingredients of onions, carrots and celery with salt pepper and water, you can add any of Parsley, Peppercorns, Sea Salt, Bay leaf, Garlic cloves, Leek tops, Vege skins, Kitchen scraps, Carrot tops.
Don't use brassicas like broccoli, cabbage or turnips though as their flavour is very strong.
This broth can be used in any soup or to make sauces or gravies.