Rinse the fish bones and place them in a large pot.

Add the onion, celery, thyme, parsley, bay leaf, and peppercorns to the pot.

Cover the bones with cold water.

Bring to a simmer and skim off any foam that rises to the surface.

Simmer for 20 minutes, being careful not to boil.

Pass the stock through a fine-mesh strainer lined with cheesecloth or a paper towel.

Dispose of the solids remaining in the strainer.