Lots of us just use commercially packaged vegetable stock, especially now that organic is available, but making your own is a good way to use leftover vegetables. It's good to keep in the freezer in half-pint or pint containers.
There are lots of candidates for the stock pot. Here are a few, though, that you should avoid:
- beets and beets skins
- turnip skins (bitter)
- most members of the cabbage family (they can overpower the stock)
- eggplant, green peppers, and leafy greens like mustard and collard (can be bitter)
Just about anything else will work. Mushrooms will impart a "darker" taste, which might be lovely for certain kinds of soups. And do use those annoying tiny cloves of garlic that you don't know what to do with--throw them into the pot skin and all.
I like to use a pressure cooker for stocks, because, as I read somewhere once, "The flavor is IN the food, not ON the food"--as good a way as any to describe the enhancement of taste. But you don't have to--just simmer gently for about an hour
I suggest leaving your stock salt-free. Then if you want to use it in a bean soup or other bean dish, you don't have to worry about salt toughening the beans and lengthening cooking time.
You can, if you like, roast the vegetables first, tossed with vegetable oil, in a 400-degree onion, stirring occasionally until they've developed some lovely brown spots. This will darken the stock and impart extra richness via the oil. This is a step I often omit, but when I'm making a vegetable barley soup that I would have used some beef bones in during my carnivorous days, it's nice to take the time.
Consider this recipe a guideline only. You can be very flexible and use up bits of this and that, including lettuce and spinach.
8 cups water
8 cups chopped mixed vegetables
2 medium onions, coarsely chopped
2 cloves garlic (whole, halved, or finely minced, depending on the intensity of flavor desired)
3 large carrots, cut into a few large chunks
3-4 large celery ribs, cut into a few large chunks
2 bay leaves
parsley stalks/leaves, left whole
sprigs of fresh or dried oregano or thyme
If you have a pressure cooker, put the ingredients in the pot, secure the lid, and bring to high pressure. Maintain at high pressure for 10 minutes. Let the pressure come down naturally (if you're in a hurry, use a quick-release method).
Allow the stock to cool slightly. Strain into storage containers, pressing the vegetables against the side of the strainer with a large spoon.