Here are the guidelines from FC magazine. If I have a stock that Is heavy on any one
flavor, I label it as such so I can use it appropriately. Sometimes, I just throw all the trimmings I've saved in the pot and add whatever from the veggie drawer without doing the saute thing they mention. I try to balance what ever is there with onion, etc. Right now, I have butternut squash trimming and seeds in the freezer, should be interesting. I use so many fresh veggies and many times have some left at the end of the week, so it's perfect for me. As I said, I just think it's a lot of fun and when you're finished, if you're not happy with the taste, just toss it, you're not out much.
Here are the guidelines from Fine Cooking magazine...
Basic Vegetable Stock
Avoid onion skins and carrot tops as they'll make the stock bitter; trimming or peeling other vegetables is optional. Scrub or rinse all vegetables well, especially if they're not peeled.
1 1/2 T. unsalted butter or olive oil
2 cups large diced yellow onion
2 cups large diced outer ribs celery
1 cup large diced leek tops
1 cup large diced fennel tops or bulbs
3/4 cup large diced carrot
1 head of garlic, halved crosswise
8 fresh parsley stems
2 sprigs fresh thyme
Heat the butter or oil over medium-low heat in a large stock pot. add the remaining ingredients and cook uncovered, stirring frequently, until they have softened and released their juices, about 30 minutes (don't let them brown). Add enough cold water to the pot to just cover the vegetables, about 4 cups. bring to a gentle simmer, cover, and cook without stirring until the stock is flavorful, about 45 minutes (adjust heat as needed to maintain a gentle simmer). Strain the stock immediately through a fine sieve, pressing gently on the vegetables. Let cool to room temperature and then store in the refrigerator for up to a week. or freeze for up to six months.
Customize your stock
The recipe above makes a nicely balanced vegetable stock that's good for almost any dish, but you can adjust it depending on the trimmings you have or the dish you're using it in. If you're making mushroom soup with marjoram it, you might add two cups mushrooms trimmings and a spring of marjoram to the stock recipe. Her are ideas for other additions. Depending on how dominant flavor you want, use 1-4 cups of these enhancement vegetables.
Subtle flavors
Use these vegetables or their trimmings with confidence
Celery root
corn cobs
eggplant
mushrooms
parsnips
tomatoes
summer squash
Swiss chard winter squash (seeds and stringy insides, too)
Assertive flavors
Use these vegetables or their trimmings judiciously and only when you want to emphasize the vegetable's flavor (for example, using asparagus-enhanced stock in an asparagus soup).
asparagus
beets and beet greens (will turn the stock pink)
broccoli
Brussels sprouts
cabbage
cauliflower
potato (may cloud the stock)
rutabaga
turnips
herbs Tender, leafy herbs such as basil, cilantro and tarragon add a decidedly fresh flavor to a stock. Add four to eight items to the basic recipe. Tarragon is strong, so use it sparingly. Woody herbs such as marjoram, oregano, rosemary, and sage are more potent and the stems can impart a bitter flavor if left in a stock, so just use a couple of springs and strain immediately.