Which vegetables to use for stock?
Typically veggies are onions, carrots, celery, leeks, garlic, shallots, etc. Throw in some peppercorns, also, and a Bouquet garni. You can add most other veggies, too, and mushrooms, but avoid adding things that give a strong (bad) flavour after cooked for a long time (broccoli, brussels sprouts, cauliflower, etc.).