Use this fish stock to make chef Michael White's delicious Fish Stew with Saffron Couscous.
Stick 3 cloves in each onion and transfer to a large pot along with 1 1/2 quarts water, wine, vinegar, carrots, celery, parsley, thyme, bay leaf, and salt. Bring to a boil over high heat.
Add fish trimming and bones and reduce heat to a simmer. Cover and cook for 1 hour. Strain stock through a fine mesh sieve lined with cheesecloth into a large pan. Place pan over medium-high heat and reduce to about 8 cups; season with salt.
Cool completely before storing.
Store in an airtight container, frozen, up to 3 months.
Store in an airtight container, refrigerated, up to 1 week.