You can use whole, fresh vegetables, but you can also make stock entirely out of vegetable scraps (peelings, stems, etc.) Also toss in anything in your fridge that looks like it’s not so happy any more.
Use the trimmings from your vegetables, including onion skins, herb stems, potato and carrot peelings–just about anything in your kitchen except sulfurous vegetables such as broccoli, cauliflower and cabbage. You want something with a fairly neutral flavour. I don’t use garlic or other strong spices, I add those kinds of seasonings when I’m actually making a dish. Anddon’t add salt. As the stock becomes concentrated, it can get unpleasantly salty.