Brown onion in butter in a small skillet.
Meanwhile, simmer the vegetables in water with peppercorns, bay leaf, and salt to taste 15 minutes.
Add onion and fish to vegetables; simmer about 10 minutes, or until fish flakes easily.
Remove fish and vegetables.
Use for another dish, if desired, or discard.
Add lemon juice, salt, and nutmeg.
Boil broth rapidly 10 minutes.
Strain again, if desired.
Serve as a clear soup.