You can improvise with your choice of vegetables, but avoid bitter ones, which can compete with the flavors in a finished dish.
3 cups coarsely chopped cabbage
2 1/2 cups coarsely chopped fennel bulb
2 cups coarsely chopped celery
1 cup chopped carrot
2 tablespoons tomato paste
2 large onions, peeled and halved (about 1 1/2 pounds)
1 (8-ounce) package mushrooms
12 black peppercorns
10 flat-leaf parsley sprigs
8 thyme sprigs
6 garlic cloves, peeled
2 bay leaves
4 quarts water
How to Make It
Combine first 12 ingredients in an 8-quart stockpot. Pour water over vegetable mixture; bring to a boil. Reduce heat, and simmer 45 minutes, skimming surface occasionally.
Strain stock through a sieve into a large bowl; discard solids. Cool stock to room temperature. Cover and chill. Refrigerate stock in an airtight container for up to 5 days or freeze for up to 3 months.