Most fish markets will sell you heads, bones and trimmings for making stock. Use those from any mild white fish; avoid oily, strong-flavored fish such as mackerel, mullet, bluefish or salmon. If you like, add a few strips of lemon peel, a few white peppercorns, and some fresh fennel or dill sprigs to scent the stock gently. You can also replace half the water with dry white wine.
Recipe courtesy of Williams-Sonoma and adapted from Williams-Sonoma Kitchen Library Series, Soups, by Norman Kolpas (Time-Life Books, 1993).