PREPARATION:

1. Rinse fish skeletons; cut out gills and discard.
2. Trim tops and roots from onions, leaving most of the dried outer skin intact; cut into wedges.
3. Combine fish skeletons and heads, onions and celery in stockpot or Dutch oven. Add water, lemon, thyme, peppercorns, parsley, bay leaf and garlic. Bring to a boil over high heat. Reduce heat to medium-low; simmer, uncovered, 30 minutes, skimming foam that rises to the surface.
4. Remove stock from heat and cool slightly. Strain stock through large sieve or colander lined with several layers of dampened cheesecloth, removing all bones, vegetables and seasonings; discard.
5. Use immediately or refrigerate stock in tightly covered container up to 2 days or freeze stock in freezer containers for several months.